I was just explaining how you can see that there have to be four brushes:
+---------------------------+
| |
| +--------------+ |
| | | |
| +--------------+ |
| |
+---------------------------+
This is your wall. Try to find two points in there so that the line between them cuts the hole. Thankfully it isn't too hard:
1st point
+---------------------------+
|
`-.. |
|
\--------------+ |
| |
`----------...| |
| +--------------
\ |
|
`-..|
+--------------------------
+ 2nd point
(Let's pretend this is a straight line

). When you can find such points, you need multiple brushes because the wall is not convex. This method works with any structure.
EDIT: these ascii diagrams are kinda hard to make
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