T = 4h
let's have h doubled
T = 4(2h)
T = 8h 
from T = 4h, it is now 8h
to get the ratio of the increase you'll have
8h/4h = 2
or T is doubled..
remember when dealing with expressions involving directly proportional variables, the change in the variable from the left would have the same effect to the right.. this is if given that it is directly proportional