Não tenho certeza se isso é possível movendo as janelas, mas se você estiver disposto a fechar / reabrir:
Da sua configuração atual:
- Close one of the files on the right (:wq)
- Move to left window - Ctrl+W then left-arrow
- :split to reopen file closed from right
Do início:
- Open first file (upper-left)
- :vsplit to second file (upper-right)
- :split to third file (lower-left)
- Ctrl+W then right-arrow to toggle to second file
- :split to fourth file (lower-right)