Adding String at specific postition of a delimiter

I have a file containing data lines:

1|14|CONSTANT||11111111||00887722||30/04/2012|E|O|X||||20120528093654-30.04.2012|STA11ACT|ddts555S||||00001|rrttbbggcc|
1|15|CONSTANT||22222222||00887722||30/04/2012|E|O|X||||20120528093654-30.04.2012|rrtha772|llkis000||||00001|AAEtbbggcc|

I want to add a string say "11" after 3rd position (after the string "CONSTANT") of the delimiter in each line if it is blank. so that the file should look like this.

1|14|CONSTANT|11|11111111||00887722||30/04/2012|E|O|X||||20120528093654-30.04.2012|STA11ACT|ddts555S||||00001|rrttbbggcc|
1|15|CONSTANT|11|22222222||00887722||30/04/2012|E|O|X||||20120528093654-30.04.2012|rrtha772|llkis000||||00001|AAEtbbggcc|

thanks.

Hi

 awk -F"|" '$4=11' OFS="|"  file

Guru.

Thanks Guru !! This works perfectly fine !!!

awk -F"|" 'BEGIN {OFS="|"} {if ($4 ~ /^$/) $4=11;print}' <filename>

This is the required solution as you need to replace only if the field is blank...