The o command open the file in the previous window.

I would:

• Hit 4Ctrl ww to visit window 4
• Hit Ctrl wt to go to NERDTree (assuming that it is on the left part) or alternatively assuming that you start at NERDTree hit Ctrl wp to switch back to NERDTree.
• Hit o to open the file

The o command open the file in the previous window.

I would from the NERDTree window:

• Hit 4Ctrl ww to visit window 4
• Hit Ctrl wp to switch back to NERDTree
• Hit o to open the file

It would be possible to create a special mapping for that if needed.

The o command open the file in the previous window.

I would:

• Hit 4Ctrl ww to visit window 4
• Hit Ctrl wt to go to NERDTree (assuming that it is on the left part) or alternatively assuming that you start at NERDTree hit Ctrl wr to switch back to NERDTree.
• Hit o to open the file

The o command open the file in the previous window.

I would:

• Hit 4Ctrl ww to visit window 4
• Hit Ctrl wt to go to NERDTree (assuming that it is on the left part) or alternatively assuming that you start at NERDTree hit Ctrl wp to switch back to NERDTree.
• Hit o to open the file