A 3 bet preflop is a reraise. So if somebody raises to $4 and you make it $14, you have just 3 bet them (because the bb counts as the "first bet"). On later streets a 3 bet is a reraise ie. bet...raise then the reraise is a 3B. A 4 bet is simply the bet after the 3B (so a re-reraise).
3 betting light is when you 3 bet with a much wider range (so suited connectors etc), and in certain circumstances when you know villain's range is wide and villain needs a good hand to continue you might 3 bet with absolute trash.
@ double-A
As zach said, it depends on ranges. If you are playing 50nl FR, then that is perfectly fine, because most ppl's 3 betting ranges are very tight, so 4 betting light is just dumb because ppl aren't making plays at you. 4 betting light is something you can do to combat light 3 betting but unless 3 betting ranges are wide you do not ever want to be 4 betting light. I don't even advise 4 betting light at 50nl 6 max, cept mb vs certain regs. You don't even have to 3 bet light in 50nl 6 max to do well (you could even get away without 3 or 4 betting light in 100nl 6 max with no problem...but you'd then be missing out on some value imo).