Your 4-bet sizing should be dependent on the effective stack size.
As effective stacks get smaller so should your 4-bet sizing, as with most raises you should raise slightly more when out of positon.
30BB 4-bet 2 x 3-Bet
50BB 4-bet 2.25 – 2.5 x 3-Bet
70BB+ 4-bet 2.75- 3 x 3-Bet