thats a great size. anything up to a 12" for life would be comfortable as a general rule. you could do 2 of the more peaceful cichlids, such as uaru A. or F. and severums. but i would suggest only 1 aggressive cichlid that gets to 12".
i wouldnt suggest anything over 12" for life because of turning space and that 4ft isnt as big as it sounds.
an oscar may work but should it get over 12", which with proper care is likely, then it would be a good idea to upgrade to a 5ft tank.
i have a 9" midas in my bowfront which has roughly the same dimensions as your tank and at 9" she is making it looking small although it is more than enough room for her.
you may or may not feel the same way when you have a similar picture in front of you. its just something to think about before you stock.