If the lightmap is essentially equal if you have 4 objects vs. if they’re attached, then it would be the same memory usage. In those cases, it would be better to have the objects as a single object, because the separate objects would increase the draw call count.
In the case of a building for instance, if you have a bunch of walls that’s a lot of surface to cover, so rather than attaching all of the walls as a single object and using a really really high resolution lightmap, it’s sometimes necessary to instead split it into pieces–that essentially means you’ve reached the limit of lightmap detail within a single object and you need to split it up to be able to increase the quality by using multiple lightmaps.
Another reason to keep things separate, is to take advantage of instancing–meaning that if you use an object more than once then you can make copies of that object and it will then keep only one in memory. That’s something where you have to consider, will the game perform better if I reduce the memory usage or by reducing the number of draw calls? Usually if the mesh is very simple, then you’ll want to reduce draw calls because the memory impact isn’t a problem, but if the mesh has a high poly count then you might see more of an impact by using copies and saving on memory.
The mistake many people make is to try and build walls/ceiling/floor out of lots of pieces, when usually those meshes are very simple and you’ll get better performance by just making the full shape and taking a small memory hit.
Another thing to consider, is how UE4 generates static lighting–it gets processed on each static mesh, which means that some lightmass settings like smoothing get calculated on the individual object rather than across objects. If you have a flat surface made of multiple objects then you can end up seeing differences in the lighting between the meshes. In those cases, you either need to add detail to your mesh on the seams so that you won’t be able to tell, or you need to make those parts as a single object.