I know that this isn’t really a UE4 issue, however Unity has a vested interest in NOT answering the question.
From what I can see of the unity EULA for the asset store, there isn’t a restriction on modifying the assets and using them in UE4, however it would be good if someone from Unreal could take a look and clarify.
If we are able to use unity assets then it provides a lot of instant content, until the marketplace is fully up and running.
From Epic’s side, there are no restrictions in that regard. I can’t say what Unity permits or prohibits though. It sounds like you’ve already reviewed the Unity EULA on this point and found that they didn’t have any restrictions.
I actually went into this on another answerhub post.
there are no limitations, but its suggested that you contact the author to be sure.
In most cases they dont mind, but some will eventually use ue4 marketplace for the same work posted on the unity asset store (with some edits) and they might refuse to currently support ue4.