Do the 4" now and plan on the change over down the road since 6" pots are now totally dated. Use all the bulbs LED 4000K thta will at least keep all the light the same The big issue with replacing is the repair of the ceiling the rest is DIYable

Sounds like it's fine in your situation to mix.

I wanted to add that while there is little difference my existing 6” cans have retrofit 5” led. I’m planning to use can-less for all future.

Just remeber that you never want intergrated bulbs since then you need to again buy all matching stuff when they die.