My first inclination would be either a ranger or a bard, depending on the focus of the other PCs, and the nature of the campaign.
A ranger can cover the wilderness stuff, and dabble in both ranged and melee combat, which is useful for a fifth man. They get a wide selection of skills, most notably Spot/Listen and the ability to track. If there's much wilderness stuff to the campaign, I'd probably go with a ranger.
If, on the other hand, it's a primarily city-based or social game, I'd probably go with a bard. I've found them to be a lot of fun, and I have a really fun bard idea kicking around in my head that I want to play. Bards don't do well in every campaign, though - people have to realize that they're backup and support characters. If the DM and the player aren't on the same page with that, and the use of social skills in the campaign, then a bard is going to make the player very unhappy. But they can be very powerful and very fun in the right situation.