Definitely left a couple out; Atlanta and Spring Hill are both very strong bands in Region 4.
Spent several years in Region 4 and am acquainted with all 3. Tim is essentially correct in what he said below. You don't say what level of school you're looking at, but Regions 21 and 4 are essentially rural, ATSSB-centric regions with a few 5As thrown in. There are NO 6As in Region 4 and just four (John Tyler, Tyler Lee, Lufkin, Longview) in Region 21. Region 3 is essentially two different regions in one; there is a large suburban side with many east-Metroplex 5As and 6As, but also a large ATSSB side that borders both 21 and 4 and is basically indistinguishable from them in terms of culture... actually, that's not true because Tim is correct that 21 is Military Band Central; there are many more corps bands in 3 and 4.
Standout bands in each region include:
Region 3 - Canton, Wills Point, several Richardson/Garland/Mesquite schools
Region 4 - North Lamar, Pine Tree, Queen City, Mineola
Region 21 - Tyler Lee, Lindale, Longview
Didn't mean to leave anyone out. Hope this is helpful.
I am a director with possible family move options wondering about these Regions. Any info would be greatly appreciated about these areas. Thanks!