Do I need anything special other than UE4 to create a landscape that is handmade every step of the way? I want to make stuff like mountains and valleys and stuff but don’t know how to do it.
You can do everything in the UE4 with the landscape tool, but it’s faster when you use programs like world machine (creates a heightmap which you can import into the landscape tool). 
Just take a look at the landscape doc: https://docs.unrealengine/latest/INT/Engine/Landscape/index.html