Shell Programming and Scripting

abcdef,11/02/2015 11:55:47,1001,1234567812345678,12364,,abc
abcdefg,11/02/2015 11:55:47,01,1234567812345678,123,,abc
abcdefhih,11/02/2015 11:55:47,1001,1234567812345678,1234,,abc
abcdef,11/02/2015 11:55:47,001,1234567812345678,1236487,,abc

abcdef,11/02/2015 11:55:47,1001,1234-5678-1234-5678,12364,,abc
abcdefg,11/02/2015 11:55:47,01,1234-5678-1234-5678,123,,abc
abcdefhih,11/02/2015 11:55:47,1001,1234-5678-1234-5678,1234,,abc
abcdef,11/02/2015 11:55:47,001,1234-5678-1234-5678,1236487,,abc

awk -F',' '{print $4}' file | sed -n -e "s_\(....\)\(....\)\(....\)\(....\)_\1-\2-\3-\4_p"

awk -F, '{gsub(/..../,"&-",$4);sub(/\-$/,X,$4);} 1' OFS=,  Input_file

abcdef,11/02/2015 11:55:47,1001,1234-5678-1234-5678,12364,,abc
abcdefg,11/02/2015 11:55:47,01,1234-5678-1234-5678,123,,abc
abcdefhih,11/02/2015 11:55:47,1001,1234-5678-1234-5678,1234,,abc
abcdef,11/02/2015 11:55:47,001,1234-5678-1234-5678,1236487,,abc

awk -F,                           ########### Making comma(,) as a field seprator.
'{gsub(/..../,"&-",$4);           ########### using global subtitutaion to change any 4 chars/digits etc to their value with - by using (&-) in $4 as per your request
sub(/\-$/,X,$4);}                 ########### When we use global subtitutaion then it will place - at last of the string then I am removing it by sub which means only single time subtitutaion, That's the difference between global subtitutaion and single substitutaion in awk.
1' OFS=,  Input_file              ########### awk works on condition and then action, if condition is TRUE following action mentioned in it will be performed. So here I am making comndition TRUE by mentioning 1 and no action mentoined so awk will take default action which is print so it will print the complete line then.