Split windows for 4 panels in i3wm

Solution 1:

I don't use i3 anymore, but as you said, you ‘can split windows only vertical or horizontal.’ So assuming you have the defaults set, to create this grid-like layout, just

1. open your first window (1)
2. open your second window (2)

+---+---+
|   |   |
| 1 | 2 |
|   |   |
+---+---+

3. switch back to your first window (1) (MOD + j focuses left by default)
4. preselect a vertical split (SUPER + v splits vertically by default)
5. open your third window (3)

+---+---+
| 1 |   |
+---+ 2 |
| 3 |   |
+---+---+

6. switch back to your second window (2) (SUPER + ; focuses right by default)
7. preselect a horizontal split (SUPER + v by default)
8. open your last window (4)

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

You have the desired layout.

(MOD is the i3 combination key you use; for example I prefer the Windows (Super) key)