I'm currently starting to form a 4e game with a group of friends and they all choose core classes, Bard, Fighter, and Psion. However we only had 3 players so I started to look for another player. I managed to find one, but he didn't know 4e so I began listing some classes to him. The only class he found interesting was a Hexblade.
My question is that if I let him play a Hexblade will he out shine the other players or will he fall behind.
FYI I plan on going to level 10.
It's fairly well-balanced for your party.
The main thing that will help avoid balance issues is that the group has no other strikers. The hexblade is a relatively weak striker, but by no means bottom-tier (vampire & o-sassin, I'm looking at you).
If you had other players using stronger strikers then there would be a real risk of the hexblade seeming underpowered. Neither bards nor psions are damage powerhouses, though, so they certainly won't threaten the hexblade's position as the party's damage dealer. The only way you should run into balance problems is if the fighter does a lot of optimization and builds for damage rather than defending.
Anything a Hexblade can do, a properly optimized Warlock can do better.
The problem is that not every Warlock is properly built and thus, for some, a Hexblade may be better than a non-optimized Warlock. Generally speaking, Hexblades are probably better for newer players (particularly in the Heroic tier) although the one Hexblade kit that does necrotic damage may be frustrating when fighting undead. Yes, that flaw can be fixed but then you are going down the optimization road (which ultimately leads to a full Warlock).
