Hmm... If I had been asked to re-create this pic, I'd have chosen a different approach...

I wouldn't have used a single terrain, but (at least) seven. Approximately as in the above pic.
Most would probably be OK at 512, except to #6 (which might be better at 1024x1024) and #4 & #7 (which could probably be reduced to 256x256).

Cover the joins with shrubbery...

This approach allows you to define the textures on the bits independently...

Just my way of working. Probably a longer method.

Cheers,
Diolma

(PS: Apologies for scrawling over your pic...)