Just to offer an alternative that also works. When ready to uppot a 4" I put it inside an 8" net pot and fill with hydrotron.
I had read previously that guys would cut the 4" off the plant then uppot to 8" and some would cut the 4" where it could expand and put that inside the 8". Last run I just put the 4" inside the 8" and ran with it. Those plants both yielded a little less than 12 zips each.
Only reason I use 8" is that it gives you a little larger foot print and more weight down low. That way the plants more stable as it builds flowers.
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Now that I post that picture it has me wondering about my current grow. I kinda forgot about the inner pot lifting and getting all outta whack. I think the cage kept that in check but I don't have cages on the current plants. Only time will tell I guess.