Roots need how much room?

I have a two gallon container my box is from dealzer cash crop 4.0. Now I have 4 pk's two are doing great and the roots are huge I was wondering will they stop growing or is it enough for the plants to continue growing the other two are coming along do I need to jus leave the two in so they have enough room or will they be enough for all four to grow nicely? Or should I get a bigger container for all the roots? I've been doin good on my nutes leaves reall y nice jus wondering about the roots tho. Any help would be greatly appreciated..

This is my baby's lol

Personally id give them there own container and build a rdwc. Check out my sig for ideas, I built one cheap as.

Where can I check it out ? And yea I have another one for its own I gotta get submersible pump for the Rez to the buckets so far now this wat I got.

Y do u think they'll stop growing if there that crowded ?