When raising the lightmap resolution doesnt work, take a look at this site: ://www.worldofleveldesign/categories/udk/udk-lightmaps-05-common-problems-solutions.php There you can find common lightproblems that appear in UDK (it’s nearly the same in the UE4)
Here are some lightmap resolutions that you can try out:
128
256
512
1024
2048