What can and can't be done in Unreal Engine 4?

3D modelling software is used to your create assets, static meshes (chairs/tables/doors…) and skeletal meshes (people/guns/cars).

UE4 is used to create the actual game, so you import the assets created in blender and setup you scene/levels in UE4 and then do things like add controls/gameplay and menus.

while you can use UE4’s tools to create walls/ledges… most people just use those to to block out there scene/level and the replace the walls/ledges with static meshes made in modelling software.

hope that helps:)