I really don't see your problem with this tank! When I started fishkeeping, all you could really get in a four foot length (at a reasonable price, anyway; I'm quite mean
) was the standard 12"x15" or the 'big' 15"x18". I've kept fish in them for donkey's years; still have one in my bedroom
When you're thinking of tank size for larger fish (that's anything over three or four inches, adult size) you need to ensure the tank is a minimum of five times the fish's adult size in length and twice in width (so, to house a foot long fish, you'd need at least a 5'x2'x2').
However, it's a good idea to leave yourself a bit of leeway as many fish will grow a bit bigger than expected, if they're in a big enough tank and well cared for, so that's why I said a six inch maximum, rather than seven or eight.
Hope that makes sense; it's good size tank, just stock in with fish appropriate in size and for your water and it'll be fine