Forget the "inch" of fish theory.
You do not need to do a fishless cycle. By moving one of the existing filters over and some of the decor, you are seeding the new tank with bacteria. However that bacteria will not be enough to support all those fish in the other three tanks.
However, you are greatly increasing the total volume of water which will dilute the wastes.
Move your fish over and watch the water parameters until the bacteria establish themselves to handle the bio-load.
You do not need to do a fishless cycle. By moving one of the existing filters over and some of the decor, you are seeding the new tank with bacteria. However that bacteria will not be enough to support all those fish in the other three tanks.
However, you are greatly increasing the total volume of water which will dilute the wastes.
Move your fish over and watch the water parameters until the bacteria establish themselves to handle the bio-load.