A lot of the meshes that come with the ue4 stuff have (and im sorry to say this) very unoptimal lightmap uv’s. you could either export the ue4 mesh and edit the uv in maya, make your own mesh and work on the lightmap or try to auto generate a new lightmap inside ue4 (there is an option somewhere in the mesh browser).
Basically you want to make sure the lightmap uv’s are on grid to get rid of most lighting/shadow bleeding.
That said, be sure ALL meshes have a higher lightmap resolution. (256/512 or 1024 if needed) though for a simple wall with proper lightmap uv’s 256 should suffice.