You can use ART for non-UE4 projects, but the UE4 EULA terms still apply. That’s because, under the EULA, a Product is defined as “any product developed under this Agreement that is made using the Licensed Technology or that combines the Licensed Technology with any other software or content, regardless of how much or little of the Licensed Technology is used” (emphasis added).
So it’s fine to do, but you have to comply with the rest of the terms of the EULA and a royalty would be owed.