In theory the models designed for v3 will work with v4, but you need to apply a layout template or otherwise re-setup the display, since the gui layout will be wrong. There are also changes to how switches are handled from V3. Specifically, each switch is now multiple inputs (a 3 way AIL-DR switch is now AIL_DR0 AIL_DR1, and AIL_DR2. I believe the firmware will properly upgrade most models, but it is not guaranteed, and I'd definitely want to re-check everything.
I don't think there should be any reason for folks to use 3.0 anymore. Version 4 has a lot better stability than 3.0 did. Of course for those paranoid folks, I can understand waiting a couple weeks to make sure there are no latent showstoppers, but I think you'll find 4.0 to be the best release we've done to date (in large part due to the awesome work rbe2012 did in my absence)
