If your budget is really tight, The Behringer HA400 is a great little distribution amp with 4 individually controlled outputs.
Only costs $30. I have two. Never had any problems. Sound is good.
http://www.bhphotovideo.com/c/product/3 ... ifier.htmlWith one of these, just hook into your r16's main outs, and use the r16's headphone outs for your fifth set.
Very cost effective.
If you have a bit more money to burn:
http://www.bhphotovideo.com/c/product/3 ... A8000.htmlAt $140, this unit will cover your needs well beyond your immediate project. It has 8 independently controlled channels, but each has 2 headphone outs, giving you up to 16 musicians you can feed. Plenty of power. Nice sound. Very elegant. Extremely easy to use. I just got one a few days ago and used it for a project for the first time yesterday. Zero gliches using all eight channels.
If you still have a tad more money to invest, add this in. It will hugely increase the versatility of either of the above units:
http://www.bhphotovideo.com/c/product/4 ... atrix.htmlAt $50, it allows you to feed your distribution amp
and your moniters, playback amps, multiple powered speakers, etc... all from your main outs of the r16. An incredible bargain.
Behringer like Zoom makes a lot of products which over-perform for their price level.. I've never had even a hint of a problem with any of these items.
Keep in mind, while the sound of these items are all quite good, they in no way affect the signal going to the r16, so save your bucks for the part of the process which is super critical - what's going into the r16. As long as your musicians have a good, clear, working signal to their "cans", that's all that's necessary to get the job done quite well.
Don't forget, all these units are designed for 1/4 inch headphone jacks, so be sure you have the right adapters for this if needed.