Walter fell 4 meters and an additional three times that depth which is 3*4 = 12 meters
He fell for a total of 4+12 = <<4+12=16>>16 meters
The eighth platform is 16 meters high and the platforms are evenly spaced so each platform is 16/8 = <<16/8=2>>2 meters high
David was located 4 meters below the 16 meter level which is 16-4 = <<16-4=12>>12 meters high
Each platform is 2 meters high so 12 meters would mean the 12/2 = <<12/2=6>>6th platform
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