I had a large number of platies in the past. If you purchase four females chances are they'll be pregnant so you may prefer to try four males -- I don't remember if males fight when females aren't present. Mine would chase and nip each other sometimes when females were in breed mode but otherwise they lived very happy, healthy lives.
We fed ours regular tropical fish flakes with occasional treats (bloodworms, tubifex, daphnia).
"There will come a time when three words uttered with charity and meekness shall receive a far more blessed reward than three thousand volumes written with disdainful sharpness of wit."