A difficult question. It's a bit like asking "what sort of car should I buy?".
In in ideal world you would be able to record all four radio mics individually and then sort it out in post. Renting a hard-drive or CF recorder might be an option, your local rental house will be able to help.
Failing that, if cost is an issue and you have access to mains electricity, it may be better to buy a small studio mixer like this:
http://www.gear4music.com/Audio_Mixe...02_Mixer2.html
It's a lot easier to use than a "field mixer" (the electric handbag you see slung from the necks of location soundies) and a lot cheaper. A field mixer which has four inputs will start at around two thousand quid.
http://www.bhphotovideo.com/product/...O442N/REG/4962