Feels like the 4th to me.
1st, original roadster
2nd, model s/x
3rd, model 3/Y and refresh/plaid S/X
4th, vehicles with 4680 batteries, (cybertruck, new roadster)
Purely going off the article here.
It mentions the sale discussions occured in 2013, and then Musk asked to run it for 8 years, or until it develops it’s 3rd get vehicle.
If 3/Y were the 3rd gen, then he beat his goal by 3 years, if the 3rd gen is the cybertruck (or similar, more specifically structural pack, 4680, ground up build), he may be a bit behind his 8 year goal.
Perhaps:
Gen 1: 2008 - 2012 roadster
Gen 2: 2012 - 2016 S/X or 2012 - 2020 SEXY.
Gen 3: 4680/structural pack, whole new ground up build vehicle (cybertruck, roadster v2, plaid +) 202x?
Certainly though I can agree with your suggestion as the S/X and 3/Y seem a generation apart, in approach and overall design, new batteries, much larger volume etc.