what you're calling hand odds, most people call equity. you shouldn't fold a draw if equity > pot odds. the rule of 4 is for evaluating flop all-in decisions, not for evaluating flop calls with money left behind (as you suspected), in that case use the rule of 2. betting or raising is different than calling because of fold equity (the percent of times villain folds). usually if fold equity + showdown equity > 50%, you should bet. money already bet is held in escrow by the dealer for the winner of the hand, those are sunk costs and should be counted only as potential winnings, ie part of the pot.