Did the Mehrunes quest and got 4 hearts not seen any since...
WHERE CAN I GET THEM??? (long lists would be swell)

They respawn at that very place.

There's one in the Companions guild hall that respawns.

Near where Kodlak sits.

Alchemists sell them if your level is high enough
Theres also another quest where you can get them but you'd have to ask about that in Cheats, Hints and Spoilers

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I will move this now.

They respawn at the shrine. If I ever find non-dungeon daedra it's usually between ivarstead and riften oddly enough, but that's just me. stick to the shrine.

Make sure you stay away from the Shrine of Mehrune's Dagon for whatever the respawn period is (it's either 2 or 4 weeks - I'm not sure) because there's a bug that will prevent the humanoids from respawning there if you visit it during it's respawn cooldown period (may have been fixed with 1.4).

Khajit caravans may carry them early on - before Alchemists start carrying them.

Enthir in the College of Winterhold will sell them 2 at a time every 48 hours. Make sure you buy something (anything) from him before you start the College's questline in earnest. There was a bug wherein (may have been fixed with 1.4) if you didn't buy from him early on you never could in the future.

The other quest you can get them from is The Black Star started at Azura's Shrine

Both sides of Azura's quest will yield a few more hearts, and the successful completion of the Labrynthian labrynth will yield one heart. Members of the College of Winterhold can learn a recipe to create them from a human heart. I would put the recipe in a Spoiler tag, but my Prima guide is in Missouri, and I am in Dallas, Texas.

One of the NPCs in the Mages College sells them, can't remember the name though.

I'd say alchemists are the most common place to find them.

I've seen them regularly at The White Phial (Windhelm) and The Hag's Cure (Markarth).

There's one downstairs in the Companions place, which has to be stolen. I believe it respawns, but can't say from experience.

His name is Enthir.

Also try google, wikia, or my personal favorite uesp.

There are a few in the alchemy area of the Vaermina quest.

And a couple in the Azura quest.

Other than that you can buy them from merchants, or if you're lucky find a few in the random alchemical satchels in dungeons.

Most easily acquired buying them 2 at a time from Enthir at the Mages College. And some alchemical vendors randomly have them.

I believe the Innkeeper in Riverwood stocks Daedra hearts.