Hello boski005,
What you are seeing is the seam that is created by baking your lightmaps. There are a few ways to reduce the obviousness of this seam.
1.) Double click on your mesh in the content browser. This will open up the mesh’s details panel.
2.) On the right side you should see a tab that is labeled lightmap resolution.
3.) Change the default value to something like 256 or 512. I wouldn’t recommend going any higher than 1024 for this object.
I am linking you to our documentation no lightmaps and how to unwrap and export them for use in UE4 along with a lighting troubleshooting guide if you get lost.
Let me know if you this fixes your issue or if you have any difficulty.