I'm wondering if I should upgrade to a 4K Blu Ray player (in preperation for when I'll go full 4K, not sure)

At the moment I'm happy with my current TV and it was the biggest I could get, so not sure I can go bigger now

What features are best to look out for (Dolby Vision, HDR?)
I heard plain DVDs/CDs will play fine, just like old BD players, so that's OK, but hoping to get a region free player
