I think sprite limits would have nixed 4-player TMNT pretty handily. 2P, though, I think is certainly reasonable. Hell, if the NES can juggle it, the PCE can.
As for Rampage, there was a thread on another forum mentioning it, and I remembered all the fun I had playing it on my Apple II. Even with the slow animation and screen update it was still a blast. So many different hazards and such. Great fun.