Consider the following hypothetical:
The 5-person party is in the Engineering District, fleeing a detachment of 12 Goblin Points (Lightly-armored Despot skirmishers) and two Goblin Scuta (Points with swords instead of spears, specialize in creating openings for other Scuta to slip in and swing). They slip into a warehouse through one of the broad double-doors, which is hanging off of its top hinge. They don't stop to consider whether the massive wooden door was ripped from its hinge during the riots following the bells, or by something inside the warehouse getting out. The party does not care about these things, because the goblins chasing them attacked immediately after a much larger battle with four Savage Elementalists and their flaming-rock minions.
The players are weary, wounded, and out of encounter attack powers and second winds. With mere moments before the goblins arrive, the players manage to hoist the door back up. Two characters hold the door fast in its frame, another player braces the first two so they won't slide once the goblins start slamming into it, and the last player, a swordmage with ritual caster, begins incanting the Arcane Lock ritual...after some clever rules-laywering where he points out that the ritual description does not require a functional door, and causes anyone other than the caster to experience the door in question as "locked." Amused by the creativity, and enjoying the tense scenario, the DM allows this.
The players are now locked in a skill challenge. In order for the swordmage to complete his ritual without being overrun, the players will need to hold this massive door up for ten minutes. Luckily, goblins are small and don't weigh very much, so the players only need to make skill checks once a minute. However, both players holding the door must make both Athletics and Endurance checks; the former to keep the door closed, and the latter to avoid tiring and suffering a penalty in subsequent rounds. The player bracing the other two can also make both checks, and a success with either lets him aid one of the players with that roll (rather than using aid another needing to target a specific player when he rolls, penalizing them if he fails). Players also aid one another; if one player succeeds on his Athletics, the other player gains a +2 to his role. However, if the second player fails that roll, the first player's endurance roll takes a -2; he's left holding the door by himself when his partner slips. So long as one of the two players succeeds on his Athletics roll each minute, they don't accrue a failure in the challenge.
The Swordmage, meanwhile, is just trying to keep his concentration with Arcana checks, which should be easy; the DM isn't trying to penalize his ritual unduly, since there's still a check at the end. Each failed Arcana check will penalize the character's final Arcana roll to cast the ritual by 2. Each time the players at the door accrue a failure in the challenge, however, the mage's Arcana roll for that minute is penalized by 5-- he's jumping in fright as the door bangs inward.
It is a five person party, and let's imagine that the players turn to the last character, brainstorming how he can help. The DM has already ruled that the door cannot support another character. He wasn't even going to allow a third, until the battle cleric's player made an excellent argument for bracing the other two. The players think for a moment (and the DM checks their passive perception) and then one notices the hooks jutting from the walls to either side of the door and yells "A bar!" The party rightly surmises that if the fifth man can locate some sort of sturdy object about ten feet long, and slip it into the hooks, their job of holding the door closed will become markedly easier. The crisis may be escapable after all...but this fifth man will need to travel alone, into the depths of the warehouse.
Originally, everything above was just going to be a small hypothetical to address a problem with a particular build, but then I became enamored with the skill challenge itself. But perhaps I can open this up to a little audience participation:
The party is level 4. As mentioned, they have not received a second rest since their last, difficult fight. All the characters in the party have expended their second winds and encounter attack powers. They are truly scraping the bottom of the barrel.
Assume that there are no hybrid classes and no drow rogues (because the game is being run by a rakishly handsome mulatto who bears a startling resemblance to this humble author), and that players need another milestone before they can use magic item powers.
What is the ideal build for the hypothetical 5th man?