They are pretty much stand alone, though 3 and 4 take place in the same world, not sure if 1 and 2 take place in that same world though.

I would suggest playing 3 first, if only so you'll get some of the awesome referances they make in 4, but it really doesn't matter.

I would also suggeset the FES version of Persona 3 over the portable, but only because of 'The Answer' not being available on PSP. (I would also suggest playing it on PS2 if possible. The PS3 download available of FES has a glitch that causes the final stretch of Tartarus to blink constantly, and can cause a big headache. If you can't get the PS2 verison of FES, and constant blinking for a dungeon is a problem, the Portable version should suffice.)