I would recommend not making the rack 4 ft wide. Melamine/particle board is very heavy, but it's also not very sturdy. I would make the rack 2ft wide or less and make it taller instead of wider. much less chance for sag.

If you're using 32qt tubs, you can make it to where either 1 32qt tub will fit on a shelf sideways or two 15-qt tubs will fit on the shelf longways. I have a rack like that and while it is heavy, it's very sturdy and doesn't move/sag. I can get you the measurements for it if you want it.

If you're building it for 41qt tubs, make it deep instead of wide and don't make it to where other tubs will fit too. It will make it too flimsy and apt to sag. I have a 41qt rack that holds 6 tubs and it works well.