That comes close to what I would suggest.
Come and see the nature.
As others write, the cities are somewhat similar and not too interesting to spend a lot of time.
Chiang Khan skywalk overlooking Mekhong over to Laos.
Khao Yai at the entrance to Isaan is worth. Petchabun is nice although just west of Isaan.
For the parks, waterfalls, Mekhong etc. it much depends on your date of travel.
Can either be crowded like mad or waterfalls drying out, Chinese cutting Mekhong low you can almost walk over.
Four weeks is really too much for just this region.
You could consider doing a whole Thailand round-trip in this time.
Do you have experience with driving in Thailand?