Only 4ft after 2 years probably means that your container is too small. If you put them in the ground, you should be getting pups much faster since the roots get to roam freely and they probably won't abort either since the ground has better moisture control.
I had mine in 2 gallon pots for a while and they grew pretty slowly, but once I put them in the ground 2 weeks ago, they really jumped in size so I don't think banana plants really like growing in pots.
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