I think this depends a lot on build details.
The highest-damage PC in my (30th level) 4e game is the sorcerer, whose at-will is a Blazing Starfall that does 2d4 (with re-rolls, so averaging somewhere above 5) plus around 50 in static mods. The lowest damage PC is the invoker, whose damage isn't very different between at-wills and limited-use powers, which tend to do a couple of dice plus 15 or so in static mods. Of the more martial PCs, the ranger generally does less damage per attack than the sorcerer but gets more attacks (due to off-turn actions) - damage tends to be two or three d12 plus 20-ish in static mods (less for each component of Twin Strike because there's no stat mod); the fighter tends to roll 4d8 (using an over-sized brutal maul) and add 20-ish in static mods; and the paladin starts with a pretty low damage base (2d8 plus 20-ish) but (due to items) has a +12 or so to damage against bloodied targets.
That makes damage averages in the neighbourhood of Sorcerer 55 to 60 (typically with AoE); Ranger 50-ish from Twin Strike (but with a more reliable floor due to two chances to land Quarry damage) plus off-turn damage of 30-ish (including more chances to land Quarry damage), but all single target; Fighter 40-ish (typically with AoE, via encounter powers); Paladin 25 to 30, but lifting to 40-ish against bloodied target and/or if he gets his frost-cheese working (mostly single target); Invoker 20-ish (typically with AoE).
The sorcerer's +12 to damage from striker class feature, on its own, is equal to about 50% of the invoker's average damage.
I don't have enough 3E experience to know how big the damage differentials are in that system. But the differences I've pointed to are pretty noticeable in play (and not just at 30th level; the pattern has been pretty constant for most of the game, although the availability of AoEs to the fighter took a while to peak).