This worked out for me:

wt -M -p "PowerShell" cmd /k "echo 1st pane"; ^
split-pane -H -p "PowerShell" cmd /k "echo 2nd pane"; ^
move-focus up; ^
split-pane -p "PowerShell" cmd /k "echo 3rd pane"; ^
move-focus down; ^
split-pane -p "PowerShell" cmd /k "echo 4th pane"

It will get you 4-paned square:

4-paned square terminal