Oh Nutty I can't say much about it, because I'm obviously only a first-timer. But I've read quite a few things on MN about very similar situations & I really don't think that you're alone.
I suppose to DH, it is all a bit 'old hat' & although he'll love the baby, the pregnancy isn't exactly a barrel of laughs - the same as we find it, really. I try to explain to my DP that there are 3 of us sharing my body at the moment - me, the baby & the pregnancy. I adore the baby, but I can't wait for the pregnancy to be gone! Unless pregnancy is one of those 'magical' times where you've 'never felt better' (), I think every woman feels the same. I'd guess that it's the case for him as well, but that he's being a bloke & not doing very well at articulating it.
All he can see is that you've already done all of this before, so it's not a wondrous new experience, but that it's also causing you - the woman he loves - a huge amount of pain & discomfort. It must be difficult for him to feel enthusiastic about that, as well as the other, more practical, reasons as to why he won't touch your bump, etc.
Try explaining to him that you know that it's all a bit crappy at the moment, but that it'd mean a lot to you to think that he was bonding with the baby a bit more. You know that he's going to be a good dad again, but he needs to look past the next few months to when he's holding that gorgeous new little girl of yours. That's what this is all about & he just needs to keep that in mind, I think.
Also, as for the 4D scan, I don't think that it'll freak DD out. Everything I've seen other people saying has been about how their young children have been really excited about the baby in mummy's tummy instead of the abstract concept of a future sibling. & if you think that it will freak her out, don't let her see it. You should go if you want to, because God knows you're having to put up with enough of the rubbish side of pregnancy to justify something nice. (& who knows, maybe DH will get excited about the baby as opposed to the concept!)