This is normal for unreal. Four minutes is still a fairly fast build time. A production level that you would find in a modern FPS would take about 15-20 minutes to build.
You could try optimising your lightmap UVs for better packing so you can use a lower resolution. 2048 is pretty big. Here's some instructions.
Render small for setup, bigger for final. The new resolution means the lightmap has a thousand times more pixel compared to the old one, so the increase in rendertime doesn't look that bad.
2048 is overkill for most models, 512 or less should be enough for most assets. 2048 is 16x the resolution of 512. Also don't use preview to judge how a model will look, preview is really only there to get a rough idea of how the lighting is going to turn out, but is going to be really blotchy with a lot of seams and artifacts.
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http://polycount.com/categories/unreal-engine
You could try optimising your lightmap UVs for better packing so you can use a lower resolution. 2048 is pretty big. Here's some instructions.
https://docs.unrealengine.com/latest/INT/Engine/Content/Types/StaticMeshes/LightmapUnwrapping/index.html
The new resolution means the lightmap has a thousand times more pixel compared to the old one, so the increase in rendertime doesn't look that bad.