I know that blasts require LoS in 4e and I feel like bursts require line of effect between the target square and the creature but one my players asked if a burst would hit diagonally around a hard corner and I wanted to say a hard no, but then remembered reading somewhere about bursts "splashing" around corners and had doubt enter my mind. Can anyone help me weigh in on this.
From my reading of the PHB, both require line of effect from their origin square, and neither require line of sight (p272):
You are probably remembering a rule from 3.5 that allowed spreads to do exactly what you describe. But 4e is a bit simpler; you only check whether there's line of effect from the origin of the spell. (I'm sure there are exceptions, but those will be explicitly called out.)