The 4-2 Rule is a way to turn the number of drawing outs you have into your odds of hitting them. It's times 4 on the flop to hit on the turn or river, and times 2 on the turn to hit your draw on the river. Example: a flopped flush draw is 9 outs. Multiply this by 4, and the odds of hitting it by the river are 36%.
Low-Stakes Games are played at a level of stakes that is above micro-stakes poker, but below the mid-stakes. Online cash games from $0.25/$0.50 up to $0.50/$1.00 (aka 50NL up to 100NL for No Limit Hold'em games) are considered low stakes. Live cash games from $1/$2 up to $3/$6 are low-stakes games.