Sibelius.

Sib is easy, intuitive, and customizable in many regards. It let's you concentrate on composing and arranging while making the technology transparent. The beauty is that you remain in the realm of being an artist and creative without having to wrestle with being a technocrat.

Like I have told all my children, "You can either build the instrument, or you can play it", usually referring to the difference between a Mac and a PC, but it is a similar comparison for applications that are geared toward a creative as opposed to a programmer. If you want to remain in the creative and make the technology 'disappear' into the background, Sib is the way to go.

I became a Sibelius Ambassador for a while, doing workshops and such when it was fairly new, but not doing that any more. All the scores on my website (www.MyOpus.com) are created in Sib. Go to catalogue and then click on a title, and go to bottom of page to see pdf output.

It is great for creating parts on the fly and simulations such as you see demonstrated on my site.

Sib does exactly what you need with both creating score reductions from four parts, and for expanding a piano arrangement into choral parts. I can give you more info if you PM me.

I've always liked Finale. I use Finale 2014.
I add voices in, and take them out, as necessary, without too much hassle. The best plan is to start with all the voices necessary, and just hide/alter them as you go. It's frustrating when you have everything looking lovely, but then suddenly remember that you need an extra staff.
Piano reductions are fairly simple to create - you just click to create "piano reduction."

I've always disliked Sibelius. Since our computer lab in our Fine Arts Center in college only had Macs, they also only had Sibelius. Thus, I had to use it for any personal arranging, but also for required compositions. I hated it.

I haven't tried Lilypond.

I usually use Notion, mainly because it has a very capable iOS app, and I can switch between that and the Notion app on OS X ($150 price tag, I think). I'm not sure, however, about those layout changes; Notion does a lot but it is not nearly as powerful as Sibelius or Finale -- though for my purposes it is more than sufficient. A nice thing about Notion is the ability to set the keyboard shortcuts/numpad to behave like Sibelius or Finale if you like. I grew up using Sibelius (I had version 1.0 way back when!), so that was convenient.

But usually, I am working in Linux, and so for quick projects I still often use Lilypond, which I really do love. I use Frescobaldi, which is a (free) "wrapper" for Lilypond and makes it quite a bit easier to use, because of the syntax highlighting, auto-completion, built-in MIDI player, score wizard, automatic lyric hyphenation, and the ability to view the code, log, and PDF all at the same time. And since the PDF is hyperlinked to the .ly file, you can just click a note and it will jump right to that place in the code. And, there are versions for Windows, OS X, and Linux.

Admittedly, Lilypond by its very nature will come much easier to those with practical experience in object-oriented programming -- particularly when it comes to more obscure notation requirements formatting. But the basics (including the need presently requested) can be mastered by anyone with some patience.

I bought Dorico (the Steinberg thing I mentioned above). The input process makes sense as a process, but it's radically different from Finale, so I haven't worked with it a whole lot. I think my next thing will be to import a xml file and see how pretty I can make it.

In college I was a music copyist and made a living copying and arranging music with pencil scores and music pen parts. Later, with my College Professor discount, I purchased Sibelius 3, back in Medieval Times (around 2001) because I was told it was more accessible if you were not a techie and were running with a Mac. I'm not a techie. I wanted a program with the least amount of technical falderal between me and the notation. I now compose directly to Sibelius using no keyboard and no middle man. Then came upgrades to Sib.6.2 and lately Sib.7.5. About 10K score and part pages later with all kinds of music I guess I'm used to it and have nothing with which to compare. Shortcuts have become my best friend. I do have this observation. Previous to the Sib. Sellout, I liked the notation fonts better. They were a bit thinner and cleaner. The current versions are still intuitive and they've fixed a lot of niggling woes, but they've made a concerted effort to PC-ifie the language and herd the Mac folks toward the PC platform. I'm not liking that. Whatever program you decide on, it's the constant use that makes the process roll smoothly. If you only use the program occasionally, get one of the simpler programs. There is less to remember. I arrange or compose almost every day so the fingers just thump out what I'm hearing. I do have one suggestion for would be composers and arrangers. Get a large monitor so you can splay out the pages without piling them. I use a Mac 15" laptop and really miss the 30" monitor, especially when editing on the road.
A woman in Voces angelorum asked me if we could record a happy birthday to her brother on her cell phone. I had a few minutes to think about it and came up with this little ditty in about 5 minutes: the amount of time it took my fingers to plunk it out. I didn't even have time to turn the sound up on the midi. Familiarity breeds. jefe

Check out the free LilyPond GUI editor Frescobaldi.

Finale, in any of its incarnations, can do this with ease. One approach, which is basically the one I use most often, is that described by CCooze above. Simply set up the score with staves for all of the various parts that will be used, and, after selecting measure to be hidden in each part, from the "Apply Staff Style to" item on the Staff menu, choose "13. Force Hide Staff, Collapse". Note: the measures don't have to be empty to hide them, which is often useful when preparing, say, a reduced conductor's or accompanist's score from a full score. This allows the most complete control over what staves are to be hidden or displayed.

A second, faster method, which I also use when I don't need finer control, is to select a staff or staves with empty measures (or even select the entire score), and under the "Staff" menu, select "Hide Empty Staves" (if later you want to "unhide", you can select "Show Empty Staves").

I took a look at most of the software mentioned and settled on MuseScore. I have been very happy with it so far an found it very intuitive and it's more than capable for my needs. The learning curve was easy (for me) and I was up and being productive in a short time. The MuseScore forums are very friendly and helpful and the software documentation has been thought out to anticipate most needs and applications. I win-win in my book.

From every perspective LilyPond is the superior piece of software. It avoids the irritating spacing and weird quirks of Sibelius, the downright wrong aspects of Finale, and many of the teething troubles of MuseScore. Can be infinitely customized, the file format is human-readable, the default appearance is far superior to any other software, and it's completely free.

While the file format is initially a problem for many, once you learn the gist of how it works it's far more powerful, accurate, and easy-to-use than manually dragging and adjusting notes with the mouse. The choirs I've worked with and all of my clients all comment on the appearance and readability of my scores far beyond the usual standard of computer notation, and these kinds of results are far easier to achieve in LP.

It's perhaps most important to understand that notation still has a human element for a reason. The default output of LP is far from perfect, even if it's better than anything else out there, and requires some degree of tweaking to look great. Conversely, many of the mainstream programs can nonetheless produce excellent results in the hands of a skilled engraver. My general consensus above all is that modern program + hands-off engraver = bad score, no matter what the tools used are.