A friend insists that the changes to WFRP in 2nd edition are comparable in magnitude to the changes made to DnD4.0 (relative to 3E).
Ah, how parlance varies.
'Edition', for most RPGs, implies 'revision' more than anything else. D&D is one of the primary exceptions, where 'edition' implies 'total rewrite' or thereabouts, in more recent times, anyhow. So, actually, it's 'modern' D&D that is one of the *only* exceptions I can think of. It's likely that I, or someone else here, can come up with a few others, but even if so, they're vastly outnumbered.
I favour the 'revision' approach, except when 'total rewrite' is in fact necessary, in order to improve the game (i.e., not just a means of making more money, without giving a damn about anything that really matters, like quality). And. . . I'll leave it there.
But anyway, yes, WFRP 2e is a case of revision, most certainly. And 4e? Nope.
I too concede that, perhaps, there is another RPG where the word "edition" means "completely new product", but so far I've not had luck finding anything, beyond, possibly, a somewhat obscure Swedish RPG that I'm in no position to verify.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.