"My impression (from reading, not as a 4520 owner) is that irt SQ it is pretty close. There may be subtle improvements for high-end systems when critical "audiphile" listening of very well-recorded material. Mebbe not. For the vast majority of 4311 owners I don't think this model is a breakthru must-have. As to whether it's worth the expense and trouble, it more or less comes down to the new features (vs loss of a few features) and what matters to you... If you have the speakers, Neo:X seems to be well regarded."
"The 4311 set a pretty high bar for SQ and the major breaktrough with DSPRC= XT32 is equal in both models."
You would be right, from what I have read. It gives some improvements, such as Neo:X, and if someone prefers that over Audyssey DSX then it will give them a SQ improvement. It contains other bells and whistles, but nothing really that would noticeably affect SQ in the real world. The cost of going from the 4311 to the 4250 is just too high for the gains. Denon did create an amazing device with the 4311.
For full disclosure, I am running an Onkyo 3010 and love it, no issues at all, and am a previous owner of a Denon 3808 and loved it too.