So you started with 2? If you have access to Twin Snakes (and not the original), I'd play it next. You need to play it before you play 4, but not necessarily before you play 3.
That said- i'd play the ones on the disk in order of release and not try to do some sort of timeline based playthrough.
Also- Portable Ops is in the main story line, but it contributes VERY little to the overall story, so i wouldn't worry about skipping it.