It's a multi-system emulator, available for PC, Mac, Linux & Amiga - it supports more than 100 different systems, amongst them there are also c16, plus/4 and c364.
The degree of compatibility seems very good to me, but since I've never owned a real c16/+4 I can't tell for sure. The main issue with the emulator is that disk drive emulation is still preliminary, so it may give some problems with exotic copy protection schemes. Anyway the .prg format is natively supported, so you shouldn't have many problems to run most of the games.
Give it a look, if you want, and feel free to e-mail me if you have problems using it. Also feel free to report any incompatibility/bad emulation problems to the authors of MESS: they're waiting for this! ;) Their addresses are on the "contacts" page of the main site.