Can I merge two heightmaps?

So I'm making a map and I started with a nice 2048x2048 heightmap from L3DT. After working on the map I realize that I could (and want to) do so much more but I don't have the space. I generated a new heightmap and imported it but they obviously don't line up. Is there any way I can merge these two heightmaps into one 2048x4096 heightmap? I'm not opposed to using other programs but I'd greatly prefer not to pay for one (i do already have Photoshop).
Thanks

 

I believe a heightmap has to be a square, it can't be a rectangle.

Also, only 1 piece terrain can have collision.

 

So can I make four 2048x2048 heightmaps into one 4096x4096 using say photoshop or paint.net? That way there will only be 1 terrain and it will be square.

 

I think so, as long as the resolution is a power of 2.

 

So I've done some editing on the original terrain obviously and I've tried to export using File>Export Terrain Heightmap but it doesn't appear to export. Does anyone know what I'm doing wrong?

 

I went to Steam/steamapps/common/BeamNG.drive/content (which is where i selected it to export) and Steam/steamapps/common/BeamNG.drive. I also looked in the level files just to make sure but I didn't find anything there either.

And I've learned the hard way to backup work regular

 

Paint.NET is not capable of exporting heightmaps in the right depth for the game to use them. 16 bit grayscale I think, and its been way to long for me to even remember if I am right or not. Someone who's more active in map creation please correct this.
If photoshop can export in the right format and the right depth, then it will work. Otherwise, it wont. Period.

Merging might also be possible in Blender, however I have not explored this side of map editing.