4'x18"x20" = aprox 75gal
So far so good with the advice offered to you...
A 1m-3f ratio with adult would require atleast a 200gal tank, preferably more so as to allow for habitat decor.
You could keep one fish solitary in a 75gal, 100gal would be much better.
Being as they can swallow other fish up to 6", your choice of tankmates is somewhat limited.
Either solitary specimen or species tank.