Although I am a demo racer and I have not raced AS4, I do know that locked tyres at the front is usually, because one either:

a) brakes are set too high
b) brakes too hard (user error)
c) tries to steer too hard (not smooth enough)
or
d) has their brake bias set too far to the front...

Do also try to avoid getting on the gas to early as this can cause some pretty huge oversteer and if you don't know how to countersteer then... eh... say hello to the nearest wall.