@little wing,
I don't have a 4K TV neither a UHD player as yet. However, one of my friends, actually my best friend, has a 50" Samsung 4K TV set and the OPPO UDP-203. I had the opportunity to watch a couple of 4K movies as well as some Blu-ray movies and one opera DVD played on the 203.
Let me tell you that I was amazed with the difference in resolution and the precision of colors compared to the results obtained with a Panasonic plasma used with an OPPO BDP-83. The OPPO UHD player upsamples the 1080P video resolution on ordinary Blu-rays and you definitely notice an improvement. DVDs almost look like Blu-rays.
You have to look for yourself. If you have the opportunity of seeing some demonstration in a specialized audiophile store, preferably a dealer who sells OPPO products if you can find one, you should see the improvement. I suggest that you bring one of your Blu-ray discs with which it should be easier to notice the difference. Note that the improvement will be most noticeable on screens of 50" or bigger.
I am sure that within the next two years or so, there will be a lot more 4K films on the market. Should you eventually proceed as recommended, please let us know your impressions.
Cheers,