I believe that the most significant difference is the newer Raincoat lens coatings on the 4200, pretty much make them fog free. Now the 3200's also have that coating, it works well and is worth having on a hunting scope. Great scopes, just too bad that the bigger ones don't have enough elevation to really get out to longer ranges.
Ian has it right. The 4000 was B&L top o d line scope. Then When they changed the name over to Bushnell they added the rainguard coating and changed it to 3200 and 4200 respectivley. I have both and think they are one of the best scopes for the money, I will buy them before i ever will a leupold. I do wish in their higher power scopes they would offer a 1/4" click option instead of 1/8" clicks but makes for a excellent precision scope for out to about 500 yards with a flat shooter.