The identity provider works with ArangoDB but only in .NET Framework, not .NET Core. We ended up abandoning ArangoDB about 1/3 into the project due to some limitations of the platform that I believe may have been resolved by now, but at the time we just couldn't wait.
Regarding IdentityServer4, I just setup an authentication end point that services desktop, mobile (cordova and native) and MVC + Angular using IdentityServer4. It took a while and there a lot of gotchas that aren't documented. If you're pressed for time, I'd almost say it might be more worth it to stick with IdentityServer3.
We barreled through and got version 4 running, but it's not as clean a solution as I'd like; but I'm -really- picky so it could just be me.
Our identity server implementation uses AspNet Identity 3 over .NET Framework. At the time, .NET Core just wasn't an option with the state it was in and I built a conversion for AspNet Identity 3 for Framework since the default one is for Core. Works great!
You should be able to use the ArangoDB AspNet Identity provider with some minor upgrades. You can find it here https://github.com/actias/ArangoDB.AspNet.Identity.
At some point I want to upgrade everything, but my time is sucked up right now by client projects :(