It certainly is playable in a 4 person party - indeed, it has a fair bit of close blasts and such, so you can probably fill the minion killing role if your party doesn't have a controller.
What type of swordmage you want will likely depend on your party composition. For example, the shielding swordmage is the most defendery, potentially able to prevent tons of damage to the party - and thus works with pretty much any party. The assault swordmage is more strikery, teleporting around the field, enforcing its mark - on the other hand, he's not very sticky (aka - no real way to keep enemies next to him), so definitely benefits from having another defender in the group. The ensnaring swordmage I'me less familiar with, but my understanding is that he's kind of the reverse of an assault swordmage - he forces the enemy to come to him. As such, giving him a rogue flank buddy might work nicely.
As a defender, its probably the least sticky. Instead, its the closest to a ranged defender D&D currently has - the swordmage tactics often involve marking an enemy, and then wandering off from it to fight something else. As such, I've heard that they can be quite effective, but can be tricky to learn how to make the best use of their abilities.