U could just cut it right off then if you ever do go 4 high order the 12 inch or so sheet and re-glue it to the old one? Pretty sure you can do that with acrylic.
^^ I would use an angel grinder to cut it from the front on stop maybe 10mm from the back wall of the tank, I imagine you will have to run a razor down the back wall of the tank to get the glue off if its even possible, Once its loss cut the extra 10mm off with a jigsaw blade. saves you putting a nice slit in the back of your tank. then we have issues...
If you do want to go all 4ft, I would weld a frame around the tanks base and top, trick is to make it a few mm to big and really pack it with a glue (not sure what glue for acrylic) then add the rods as wednesday suggests. I dont think the tank will pop then! else the acrylic is very under sized and the pressure causes the acrylic to crack xD!