I'm sure there will be speed tweaks and other such improvements but I think if you're happy with the PS3 as a BD player so far no reason to upgrade, especially if you don't play many games. One thing I really like is how nice and fast the Netflix app is compared to the other platforms out there. I like it even more than using the website. BD playback is very fast as well. I'm sure someone might have a reason or two they believe you would appreciate the PS4 more but until they get in the wild it's all speculation. Maybe wait until it launches and people get time with it before making a final judgement?
I would also look at what high end smart blu-ray players have to offer. If you're not really into the gaming part of it, you might be able to get something comparable (if not better) for half the price the PS4 will cost.
I just picked up a mid-level Sony smart BD player (S3100) for my bedroom that does everything my PS3 does except games and 3D (which I didn't need), plus I get Pandora, Slacker, and dozens of other apps the PS3 doesn't have, for only $100.
Need a bit more info, but there are rumors of 4K scaling. There's support for 4K in the system, but they haven't talked much about how or what it applies to yet (digital media / PSN games / scaling 1080P BD?)