i just orderd razrs 20x11x9

6 ply is a heavier and thicker material then 4 ply. The bigger then number the thicker the material. For trail riding i would get as thick as possible, but for racing run the thin stuff for weight.
 
Does your blasty have 9" rear wheels? If they are stock wheels, they are 8". If you dont have 9" rear wheels then its time to hunt down some warrior/banshee hubs and rear wheels.
 
the 6ply tire is more heavy duty (and heavier) and will be more durable. i ordered a set when they first came out and got the 4ply as the 6ply was not yet available and mine have been great and i have yet to get a flat (i also have tire slime in them). when i get my next set i doubt if i will get the 6ply tires as mine have many years on them- mountains, gavel/rock pits, dunes.
 
I got 4 ply Fast Trekker and they are very good. 6-ply are basically going to be harder to get holes in but my 4-ply are holding up through some tough terrain.