As I mentioned earlier, I have rev 2.7, which is the first one I tried that got the picture right (using HDMI -- correct color decoding, picture processing, etc.) The only disc I've had a problem with since is Gothika HD DVD and that wouldn't play on my RCA either (again,the only one) so I'm satisfied the disc was defective.
Given the way the player functionality varied somewhat with every release (color and picture processing as well as HDMI/DVI and disc compatibility), I chose to stay with what I knew worked well for me (very difficult to back up). So, 4.0 is the latest, but I wouldn't presume it to be the greatest. My suggestion is not to go beyond 2.7 unless you've a specific problem you're satisfied it will correct.