Patch 4.0.1 went live yesterday, and I think GetActionInfo now behaves a bit differently; its use is currently causing errors in Dominos (Dominos_Buff, specifically) and some testing of my own shows that the its return variables have changed. My testing says this is the new behavior:
type, globalID, subType = GetActionInfo(slot)
In other words, spellID (previously arg2) seems to have been swapped for globalID and then removed entirely (there is no longer an arg4).
I'd update this page, but I'm new to editing pages here on on the Wiki and, besides, I'd like to see if anyone can verify my "findings."