Pros use whatever tools they can to get the job done. It depends on the kind of results you want.
If you want realistic or semi-realistic landscapes, then you’re going to want to look into a program like World Machine or TerraGen to generate your height-maps. You can then import them into Unreal, and edit them in the editor as you see fit. (Probably the best option, and the one I recommend.) There are a lot of tutorials about World Machine & UE4 so there won’t be too much of a learning curve for that. Not sure about TerraGen & UE4 as I personally haven’t used it yet, but there shouldn’t be too much of a problem getting similar results.
If you need something more cartoony, then you would probably get the most out of just painting the landscape in the editor.
Of course, either way, it’s entirely possible to paint a basic landscape in UE4 that works for your needs, then export it, bring it into a program like World Machine, make your changes to make it more realistic or add weathering/erosion, then re-export it back into UE4 and then paint the final touch-ups.