which sentence(s) are preferable ?
1. Girls run slower than boys.
2. Girls run slower than boys do.
3. Boys can run much faster than girls can.
4. Boys are sometimes clever than girls are.

Question

Answer

For the first there it is all a matter of emphasis. Are you trying to emphasize that girls are slower or boys are faster? Whichever one you use as the actor (or subject) will receive greater emphasis.
Example: 
"Boys run faster than girls"  focuses more on boys
"Girls run slower than boys" focuses more on girls

Finally, number 4 would be written " Boys are sometimes more clever than girls"

Hope this helped :)

Answer

Number 1 , number 2 and number 3 are perfectly acceptable, for number four I'd say( and most people in Ireland would say) Boys are sometimes cleverer than girls.

Answer

depends on what you're trying to say. to express the general idea in the first 3, i'd say 'boy run faster than girls'

for #4, 'boys are sometimes more clever than girls'.