As far as the underneaths/mechanical designs etc..., the table mechanics would be better on answering those points of view. But as far as play ability, the GC IV is my personal favorite table of the bunch. I also love the flush pocket design on the IV. The rubber on the back/top of the pocket was a bit obtrusive to bridging etc... on the older models.
Actually, I would be one (of a few I guess) that prefers the GC IV over the Diamond tables. I agree much more with what goes in the pocket and what doesn't on the IV, as compared to the deep shelf design of the Diamonds (both tables having the same pocket sizing 4 1/2"). I see one no easier or tougher than the other though, just different.
One thing I do miss on the GC IV though is the counters on the end rail for Straight pool. I guess during the GC IV's production 9-Ball was king, and the counters were not needed as much as on older models (or so Brunswick thought). But I would give up counters any day for the flush pocket design of the IV.