This seems like a homework assignment, where you are supposed to design a 4-bit processor, otherwise there would be absolutely no reason to use a processor as you've described. You should do your homework instead of asking for code.
If you already did it and wanted to compare then post your code and you're certain to get lots of critiquing. Otherwise take a look on open cores and look at some of the simple 8-bit processors and extrapolate a 4-bit one from that.