on the flop if u have four flush,that means if ur flush is good(exp NUT flush) if in worst case the other guy had a set u have almost 40% of winning by your flush,but on the river u have about 20 percent mux.that means u should alway play flush to the turn,but on the river no.
on the river it is not a firm no,u should consider pot odds and implied odds as well and know what cards ur opponent have to count your outs and make a decent decision .