I paid under $160 for my DMPro (as the result of a dying LCD...but so far it's still working, just dim in a rarely needed area--half of the 2nd digit of kit numbers)
and I am a very happy camper. I was unsatisfied until I read the manual. It took reading through that to get things set up properly for me. But now I have a pretty basic idea of how to fix any issues I might have with it, edit sounds to my liking (or to fit my needs), and I think pretty much use all of the features the module has to offer. (Though I'm still a little lost on effects, I'll need to give that section another look)
but I have come to the decision that if I ever found another DM Pro, I would buy it. Just to have a back up. That's how much I like the module.
Unfortunately, I've never used a D4 so I can't advise in particular against that. I can however say that my DM Pro is FAR less buggy than my DM5, it also has an amazing (to me) feature set.
If you're willing to deal with the learning curve, I say go for it. I really don't think it's as complex as people make it out to seem. Granted I've only ever used Alesis modules (DM5 and DM Pro)
if you get the Pro and don't like it, I'll totally buy it from you