I can definitely see the difference having more root space in the 5g compared to the 3g.
My 3g plants tend to be shorter single cola plants but when I use the 5g giving them alot of room they explode with side growth during the stretch.
You will be able to get away with the 4ft in a 3g... ive done it before but your yield will go down. It will go up if you switch to a 5g.
DISCLAIMER: everything I post and say are lies. I am simply roleplaying. All photos were found on the internet and/or fake!! I do not grow or use anything illegal.