shell - Repeatly replace a delimiter at a given count (4), with another character -

given line:

12,34,56,47,56,34,56,78,90,12,12,34,45 

if count of commas(,) greater four, replace 4th comma(,) ||.

if count lesser or equal 4 no need replace comma(,).

i able find count following awk:

awk -f\, '{print nf-1}' text.txt 

then used if condition check if result greater 4. unable replace 4th comma ||

find count of delimiter in line , replace particular position character.

update:

i want replace comma || symbol after every 4th occurrence of comma. sorry confusion.

expected output:

12,34,56,47||56,34,56,78||90,12,12,34||45 

using awk can do:

s='12,34,56,47,56,34,56,78,90,12,12,34,45' awk -f, '{for (i=1; i<nf; i++) printf "%s%s", $i, (i%4?fs:"||"); print $i}' <<< "$s" 12,34,56,47||56,34,56,78||90,12,12,34||45