More generally, don't worry about trying to get more XP by going with a smaller party. The campaign is pretty well balanced for a party. Bring in your full four characters and grow them. If at some point you want to go with a smaller group, you can always leave the spares behind - there are no forced party members, unlike the OC.
Someone mentioned you should have a ranger or druid. Well, maybe. First, there are a druid and a ranger NPC available to join almost from the start, so don't let that deter you from your personal preferences. Second, I wouldn't even put a druid in that "must" category - a rogue is better. Stealth and searching skills are what's called for; the only thing a rogue misses out on is faster overland movement, but he can put points into Survival skill and do alright. Also, a rogue will turn up more and better loot, which matters early before your ship comes in (okay, make that "before your fleet comes in"). Possibly consider a rogue/ranger hybrid.
Someone also suggested making your stealth/searcher your lead character. Again, I differ. While it is no doubt helpful at some points, those points are rare and can be avoided. The forcing of your lead character onto the overland map only happens when you come out of a significant encounter area (such as a town or monster labyrinth), so make sure there aren't throngs of monsters wandering nearby when you go into such an area; also, remember that there are ways to evade undesired encounters; and make use of the plentiful Stones of Alarm, and never go out onto the overland map at death's doorstep.
On the other hand, non-hostile interactions happen all the time; I always put a charismatic spokes-model type in the lead position to save having to switch (literally dozens, if not hundreds, of times). That character will have to sink some skill points into non-traditional Skills (spot/listen and survival) and should have a couple of Feats (Point Blank Shot and Weapon Finesse/Sneak Attack) which are probably useful anyway; that's if you want to take full advantage of the Adventurers' Guild.
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p.s. The opening scene can be very rough without a full party. Since you want to be perfect, know that there are bonus XP and a Feat available for getting through it unscathed.
p.p.s. You won't be "perfect" in any case - that requires a gimmicky character to get the "Favored of the Road" Feat.
p.p.p.s You're not going to be anywhere near "perfect" without a rogue, in any case; and there isn't a handy NPC rogue available until halfway through the game (and she's not that great without some remedial training).