You have to wait a few days between parts III and IV. Just an FYI.
Here we are again, Arniel is utterly useless without us. Once again, he will ask us to go speak to Enthir on his behalf, who has another item he needs. Agh, I would have put Arniel's head through a wall by now...
Go speak with Enthir again. Enthir will get short with you (as usual) and tell you that the courier NEVER arrived. AND it was Arniel who set up the arrangements, so he doesn't even know what item it is that Arniel is having brought in. Our new objective will be to " Find the Courier ".
Thankfully the game will be nice to us and will tell us where the courier is on the map. The game will give you a RANDOM LOCATION to search for the courier at (I know this for a fact because it sent ME to Mara's Eye Pond/Den, which I have already cleared out).
Head to your location and scour the place high and low for the (dead) courier. You'll find a dagger named " Keening " on him. Grab it and return to Arniel.
You'll now just need to watch Arniel do his experiment. It seems like the whole thing was pointless until... well, there's no more Arniel. You are left all alone now, but you do get some rewards: the dagger Keening is yours now as well as a unique Conjuration spell: "Summon Arniel's Shade" . PLUS, you can rest easy knowing that Arniels has no more tasks for you!