Putting the differences in the amp sections aside, the x4000 will lack FREE Amp Assign and 11 channel processing. The x4000 is made in China and the 4520ci is made in Japan. The build quality on BOTH units isn't even going to come close to the 5308ci though. I kept my 5308ci and just paid the $1000 to upgrade it since I'm partially using the AVRs amps to drive some of the surround speakers. Where you'll benefit by getting the x4000 or 4520ci is 4K, 3D, and connectivity/streaming apps -- something your Oppo player will be able to do but you'll have a more complicated setup.
The 5308ci still fetches decent money on eBay, etc., so you'd probably be able to sell it, buy the x4000, and STILL have cash left over.
HD-DVD = 94
Blu-Ray = 120 ( 24 Warner red2blu )