To change pane 4 to pane 3 after pane 3 exits:
C-b { move the current pane to the previous position
Here are more shortcuts for moving panes around:
C-b } move the current pane to the next position
C-b C-o rotate window ‘up’ (i.e. move all panes)
C-b M-o rotate window ‘down’
C-b ! move the current pane into a new separate
window (‘break pane’)
C-b :move-pane -t :3.2
split window 3's pane 2 and move the current pane there
Source: tmux cheatsheet