I don't think outs 3&4 can be patched to the Monitor Mix, therefore you'll only hear playback over them. I suppose the intention is to have monitors running off 1&2 while 3&4 are free to use as aux sends to an external FX or somesuch, at least that's how I'd employ them.
If you wanted to use 3&4 as a seperate monitor feed, you'd have to use a program that supports software input monitoring, although there is extra latency delay with that as the audio has to run into the sequencer and back out again.