The Barbarian can hypothetically act as a defender. One possibility is a bard as a leader/healer, thanks to his ability to turn the barbarian into more of a defender with misdirected mark. A shielding swordmage could work like a paladin by absorbing damage in place of healing.
Either way, the group is going to lack certain things. Without a defender, the monster is going to be able to go after just about any party member they want without much difficulty (the melee characters can cause some trouble, and opportunity attack abilities might be more likely to come into effect). Without a leader you'll need to make everyone else as tough as possible (and/or make sure the defender is VERY good at their role of being tough, but also drawing attacks towards him). You'll also be missing out on some of the boosting effects of leaders (having humans is helpful though, as they can get feats to give themselves those kinds of boosts). Luckily the barbarian and the ranger are self sufficient as far as strikers go (opposed to a rogue that needs to get combat advantage). The wizard is going to be the main person that has to be protected, but he can try for controller effects more so than being another pseudo-striker. Effects, like the illusion based at-will [one of the first 4e class act articles] can help replace a leader by debuffing the enemy (since there are less powers to buff the party).