If he Transcended Divinity then wouldn't that put him in Overgod territory?
Perhaps. I mean, if you talk with the people that played with Gygax, they will say that Gygax's Tharizdun was an Overgod (that is why in older D&D websites he is listed as an Overgod). I guess this alternate Tharizdun is a canon version of Gygax homebrew Tharizdun.
4e is sorta like Dark Souls 2. Technically, it isn't bad. It is well made. Most of us (myself included) find it to pale in comparison to stuff like 3.5e (my personal favorite).
It's ok. I mean, in the years I've been playing, I've known people who thinks the same about 3.x. For them, their fave is 2e. Then, others says the same about 2e, because they like 1e or Basic, or 0e, or Holmes... I guess that we can coexist, nonetheless.
In the end, the lore is the same. I mean, for instance, by the end of the edition they revealed that most of the Blood War stuff and the Orcus stuff (the Shadow that Was) and Vecna stuff is canon for 4e. So yeah, those demons that helped Tharizdun with the Shard of Evil are in fact the "loths" for instance (seems they planned all of this and all "is going according to keikaku").
Anyways, does that mean do you need more feats from 4e? I mean, I have a few ones left. Mostly the stuff about a primordial who spawned the modrons vs Mak Thuum Ngatha, Tharizdun's feats (he has a few on his own), the Abyssal Plague, and a few other stuff.