Upload & Sell: On
Charlie -- For the kinds of subjects you are interested in, you will benefit from FF over APS-H.
I recently used a 1Ds3, and while it takes very nice pictures, for precision LV focusing, it sucks. It isn't really capable (because of the LCD) of resolving ultra fine detail for focus. My 60D is better.
I recommend you keep your 1D4 (which is a brilliant camera), but add the 5D2. Don't worry about build quality, the 5D series is fine for your uses (except for pouring rain). You are right to worry about improved IQ, and the 5D2 has that, plus the better LV, better use of WA lenses, and industry-leading video.
Personally, the 16-35L II is one of my favorite lenses, and I love to use on the 1D2 where it works out to 21mm to 46mm. But that is a long way from what I get when I use the 1Ds2 where it is a full 16-35. Every wide angle mm makes a huge difference at the widest end. Even 16 is noticably wider than 17. Why loose 5mm?
Your 1D4 is a gem, but you need another body with a step up in angle of view and MP quality = 5D2. (original 5D has no LV, and only 12.7 MP -- hard to recover from that!)