4 foot tank. This sound like a standard 55 gallon tank ... 48X13X20
55gal of water weighs 458.95 pound (55 X 8.345). If plywood can support this weight (maybe little more with substrates and other equipments), then go ahead.
Other standard aquarium with 48 in length are 75, 90 and 120 gal, in which case weighs more with water than 55 gal tank.
In my exprience, plywood isn't simply strong enough to withstandthis kind of weight for an extended period of time.
Rohn