If it ain't broke . . .but sometimes, hibiscus like to be alittle crowded, however I would pull them gently out of the 4" pot and see if the roots are in a tangle. If so, then I think you little buddies will need to go there separate ways and will most likely benefit by having there own 4" pots.

Ok, I'll leave alone. One is much larger than the other (different types). Isn't the general rule of thumb height twice the diameter of pot is when to transplant. So when they get 8 inches high...

Sagebrush