I don't know for sure why they skipped Disgaea 3 and 4 on PC but I have a very strong suspicion, and the culprit is the Playstation 3.

You see, Disgaea 3 and 4 were originally a PS3 exclusive, which was infamous for being hard to code. I suspect making those port would cost NIS more than they are willing to pay so they just... skipped it.

As far I know this is the same reason why Demon Souls was never ported to PC too, which is a pity because it's the one game of the Souls series I enjoyed.

p.s. this is just a theory thou, like some people already pointed Sony paid for Demon Soul development and NIS may just want to release Disgaea 5 while it's new and more relevant.