I’m guessing you’ve got things sorted out by now but I wanted to point you to my screenshots gallery of Far Cry Primal. I’m also working on an Island level and as such I was taking reference screen shots of how they’ve placed the meshes in Primal so I thought it might help you.
Also, I suggest searching UE4 Speed Level Design on youtube. You’d be suprized at how infrequently people actually use landscapes to build out most of their playable area. Here’s a great example: https://www.youtube.com/watch?v=_8MrxjgxWWs
I started out like you thinking that you’d use landscape to model out the entire world, but I later realized that landscapes are best used to simply form the ground that you walk on and modular rock meshes work better for actually forming the cliffs and tying it all together. World Machine is better for generating distance mountains that you never actually get to or adding some extra details to the parts you’re walking on. You can also use it to create some pretty awesome modular cliff pieces that you can then use like the guy does in that speed level design video. Here’s a tutorial that I made for creating quick vector displacement stamps using Mudbox and World Machine https://www.youtube.com/watch?v=vkb9HX6FSZE
Might I also suggest the following Marketplace packages as I just bought them myself and they’re brilliant:
Don’t bother with the speed tree licence as you’ll have to learn the software and it only comes with limited set of textures for making trees. Save yourself time and money by purchasing high quality assets from the marketplace.
Oh, and finally I recommend the free version of L3DT as a really fast way to generate a base terrain for you to work from. L3DT - Large 3D Terrain Generator