#!/bin/bash
# First 4 occurrences:
echo 'one two three four five six seven' | \
sed 's/[^[:space:]]*/\n&/g;:t;/\n/{x;/.\{4\}/!{s/$/./;x;s/\n[^[:space:]]*/TEST/;bt};x};s/\n//g'
nl ='
' ; sed "s/[^[:space:]]*/\\$ nl&/g;:t${nl} /\n /{x;/.\{4\}/!{${nl} s/$/./;x;s/\n [^[:space:]]*/TEST/;bt$nl };x$nl };s/\n //g" <<< 'one two three four five six seven'
IyEvYmluL2Jhc2gKIyBGaXJzdCA0IG9jY3VycmVuY2VzOgplY2hvICdvbmUgdHdvIHRocmVlIGZvdXIgZml2ZSBzaXggc2V2ZW4nIHwgXAogIHNlZCAncy9bXls6c3BhY2U6XV0qL1xuJi9nOzp0Oy9cbi97eDsvLlx7NFx9LyF7cy8kLy4vO3g7cy9cblteWzpzcGFjZTpdXSovVEVTVC87YnR9O3h9O3MvXG4vL2cnCgpubD0nCic7IHNlZCAicy9bXls6c3BhY2U6XV0qL1xcJG5sJi9nOzp0JHtubH0vXG4ve3g7Ly5cezRcfS8heyR7bmx9cy8kLy4vO3g7cy9cblteWzpzcGFjZTpdXSovVEVTVC87YnQkbmx9O3gkbmx9O3MvXG4vL2ciIDw8PCAnb25lIHR3byB0aHJlZSBmb3VyIGZpdmUgc2l4IHNldmVuJw==