b4.1 has aluminium rear hubs (and larger outer bearing), b4 has plastic.
b4.1 has v2 shocks (o rings load from bottom with threaded cap)
b4.1 has revised rear shock tower.
b4.1 has updated cvd's
Both have ball diff.
Shocks are the biggest improvement.
b4 is still a great chassis (I run the ol' girl). I'd like to upgrade the shocks and get the +8 chassis. Other then that, the buggy is great.
Phill