#!/bin/bash
sed 's/\(\([0-9]\+,\)\{4\}\)\([0-9]\+\),/\1\3\n/g' <<FINISH
1,2,3,4,5,6,7,8,9,10,11,12,13,14,15
FINISH
IyEvYmluL2Jhc2gKc2VkICdzL1woXChbMC05XVwrLFwpXHs0XH1cKVwoWzAtOV1cK1wpLC9cMVwzXG4vZycgPDxGSU5JU0gKCjEsMiwzLDQsNSw2LDcsOCw5LDEwLDExLDEyLDEzLDE0LDE1CgpGSU5JU0g=
1,2,3,4,5
6,7,8,9,10
11,12,13,14,15