The Steam sale makes me want to buy obscure stuff that everybody mentions, but I have never played and feel like I should. All the Phantasy Star Sega genesis games are in the Sega pack (I dont want the whole pack) and I know that Jeff has a real like of the series, or at least the online ones.

My question is whether I can just buy the last one, since it has the highest general reviews, or if it won't be as good in that context because I don't know any of the history?

I'd say yes. There are a few neat backstory bits that if you played II or III you'll get, but as a whole each Phantasy Star game is very well contained.

And if you play one, play this one, as it is easily the best of the series.

I'll second that yes. There are some references you'll get only if you've played the prior three games. But the game stands alone, too.

Also, the game is set so far from the other games temporally that you really don't have to worry about them in the story.

Yes. I started with PSIV, was obsessed with it, and gradually worked my way backwards. Then forwards again.

The connections are definitely there - there's even a brilliant nod to PSIII, which was almost entirely removed from the rest of the series - but nothing that would confuse a player who starts with PSIV. Also, PSIV is by far the most accessible and polished of the original series, and the best game of the four partially for that reason. It's easily one of the two or three best RPGs of the 16-bit generation, so if trying to play I - III first puts you off the experience of IV, I'd say forget about the earlier games entirely.

PSIV needs to be experienced; the rest I consider wholly optional.