Unless they've added more cameos in Golden, the only Persona 3 cameo in Persona 4 is Chihiro, who's a minor social link character and you won't miss much by not knowing who she is.
Persona 3 Portable has a cameo from a younger Yukiko if you play as the female protagonist. If you play through the game with the male protagonist, you have to settle for a cameo from Noriko Kashiwagi, who's a teacher and minor character in Persona 4.
And yes, Arena connects both games (but with the focus on the Persona 4 cast), but that's pretty irrelevant, since it was released after Persona 4 anyway.
Edit: This is really uninteresting, just play Persona 4.