Just wondering, is there any way to import an Unreal 1.5 level into UE4?
No, they are not compatible.
The long version: not directly, no. You’d need to convert the whole thing into intermediary formats and then move those into UE4. Additionally, things like pathfinding nodes, lighting, weapon placement and any level logic would need to be recreated entirely from scratch and materials would need to be set up for every texture. Also note that Unreal 1’s renderer wasn’t built for physically based shading so you’d need to rework virtually all the textures to leverage the different shader model.
The short version: kind of, but it’s not worth the effort.