I'm of mixed feelings on this. I'd agree that if you want to make a weak character, just make them lower level. Or, since 4E seems to specialize equipment by role, have them use weapons/armor that don't work for their class (like a Rogue using a greatclub and heavy armor).
Also, there's an important distinction between a character who is actually weaker (accomplishes less, more likely to die), and a character who seems weak, but manages to accomplish a lot and escape death frequently through amazing luck. If you're "lucky" enough to slay Ogres, your stats should have a high enough attack and damage bonus to actually accomplish that. If you're "lucky" enough to survive an ancient dragon's firey breath, then your Reflex save and HP should be high enough to do so. You can still play the character as untrained, incompetent, and so forth - just make the stats reflect your actual performance. For instance:
Char 1: Attack bonus +15 - 70% skill, 30% great strength
Char 2: Attack bonus +15 - 10% skill, 90% luck
However, I do somewhat miss the diversity of gameplay in 3E. Playing different classes was actually a different gameplay experience. An experience that could have been balanced better, but that didn't deserve to be thrown out wholesale.
And it's by no means impossible to balance mechanically different systems. One game (that's in progress now, I believe) uses the same set of components to build spells and martial attacks, but then differentiates them by how they're used. Spells are twice as strong for their level, but take two rounds to cast. Right away, this adds a different strategy, and brings to mind situations where spells would work better or where martial skills would work better.
There do seem to be a few touches of this (Fighters seem to get less encounter/daily exploits, but they aren't wasted if they miss), but personally, I'd prefer more variance.