How can i make 4 neighbors terrains in one terrain?

Hello everyone, I’m making a game for mobile devices and because I don’t know what the “neighbors terrains” tool is, I made a map from 4 neighbors terrains, after testing I found that FPS became smaller due to the large number of terrains. I decided to make one terrain but I don’t want to remake it from scratch. I want to export .RAW elevation maps from those 4 terrains and import them into one terrain. How can I connect 4 height maps. pls contact with me sas.pushok@gmail.com or VK https://vk.com/offen_joys
into one?

 

The quickest route I can think of is to open the images in a photo editor and scale them down to create one image as needed. I use GIMP2 for my image manipulator.