vim: open 4 files split horizontally AND vertically

On the command line, vim can open 4 files split horizontally,

vim -o file1 file2 file3 file4

or 4 files split vertically,

vim -O file1 file2 file3 file4

How do I open 4 files such that they are split both horizontally and vertically, like this?:

-----------
|    |    |
| 1  |  2 |
|    |    |
-----------
|    |    |
| 3  |  4 |
|    |    |
-----------