My opinion:
I HATED the RB1 guitar with a passion. Buttons felt way too big and cheap. Hated the mushy strum bar. Never tried the RB2/3 guitars. The GH:WT was my favourite guitar, and I went as far as buying a second hand one on ebay before release to use. The seller sent me the wrong guitar, so I returned it and decided to just try the RB4 strat.
I'm very pleasantly surprised. I don't know what it is and I'm not sure if/how it's changed from the RB1 guitar, but I actually like the softer strum bar now and I would go as far as saying I prefer it over the clicky version. The buttons feel slightly smaller and better made than RB1, definitely an improvement, but I still do prefer the GH buttons as I find it easier to move between quick HOPO's with them.
If the RB4 and GHWT guitars could have a baby, that would be awesome.