A 4 foot tank should be fine for a few juvenile discus or a breeding pair but if you have room then bigger would be better long-term for a group of discus.
Although discus have been successfully kept in harder water, in their natural environment (Amazon and other rivers of northern south America) the water is acidic (below pH7.0) with little or no water hardness.
Many successful breeders & exhibitors swear by beef heart, although the flesh of warm-blooded animals is fundamentally bad for fish and difficult to digest properly.
This type of feeding may enhance fish growth and colouration but it is totally unnatural.
Discus are mainly herbivores (plant eaters) and the stomach content found in dissected deceased wild discus contained detritus and plant matter, decapods, wood matter, bugs & crustacea. Also no fish meat was ever found in the stomach of a wild discus.
It is suggested to feed things such as spirulina, spinach, peas, other vegetables, brine shrimps, bloodworm and krill as they are more natural and easier for discus to digest.
Hope it helps