My considerations on the echoes of source 4.
First if the World was perfect the location of the echo wall would be in the ellipse where the source and the receiver are the locus of that ellipse. Because the ellipse has the propriety of been the geometric place were the sum of the distances to the locus are constant. Here is an explanation of that.
In the 1 to 3 shots the echo is around 0.08s. In the 5 to 8 is 0.17s
I manage to estimate source 4 position in both times.
Here is the ellipse for shots 1 to 3.
Here is the ellipse for shots 4 to 8.
These are not perfect conditions here because there is no line of sight from the source to the receiver or no line of sight from the source and the receiver to echo wall.Therefore, the probable location is in within the ellipse.
For me I’m inclined to believe that is the represented blue walls in the images.