In an single raised hand the preflop aggressor is the player that made the raise while the defender is the player that called. It is an important distinction since being whether the aggressor or the defender changes the post flop strategy of the player.
If, for example, Hero sees a flop in position and gets checked, the percentage of times hero will bet are different depending on whether he/she was the preflop aggressor in which case his bet will be called cbet.
Since there can be re-raises, in the broad sense, the player that made the last raise pre-flop is considered the aggressor but to differentiate an standard hand from a 3betted hand in this last case the aggressor will be nominated as the 3better reserving the term “preflop aggressor” for standard hands only.
The aggressor could also be a 4better but once a player gets 4betted there is no option to call, the player should either fold or push all in. Since the purpose of knowing who the aggressor was is to determine the strategy to apply on the following streets the term 4better will rarely if ever be use to post flop strategy analysis and more likely be use to refer a player that 4bets a lot.