4MP is the new 1080p, should be standard resolution for most people setting up new systems... 1080p is the old 720p, and 720p is now so bad its worse than analouge.
Technically anything >1080p is considered Ultra High Def (UHD)
4k is 3840×2160, which is still far away from 4MP 2688×1520.. you need 8MP+ to achieve 4K UHD.
prices are not there yet, maybe in another couple years.. h265 alleviates most of the storage issues with UHD so going up a generation wont be a big jump in recording storage requirements.
If you buy a new NVR today knowing whats on the horizon, your wise to get one with h265 support.
I have zero interest right now in 8-12MP cameras for residential use right now, but I am upgrading my fixed cameras to 4MP mainly for the IVS features.. not the pixels, but they are nice too.. these 4MP cameras got faster underlying hardware and that allows for more video processing features like 120db+ WDR, Line Cross, Object Tracking and a better overall feel.. its not a giant leap from 1080p to 4MP but it kinda feels like it.. I think alot of 2MP 1080p cameras were pushing the hardware of the time, they got slow and unresponsive as you cranked quality up... where as these 4MP cameras feel like they have ample overhead to do even more.
I want to see starlight/darkstar cameras with big sensors and great low light to come down to earth before 4k honestly.. I'll buy all new stuff when it does.