Hi Nandy,

Please always use code tags as per forum rules for commands.
Following may help you in same.

EDIT: Edited the post as previous post was not giving the requested output. Thank you Scrutinizer for pointing out the same.
Not a best of the solution but will provide output for input provided by the user.

Code:

awk  '{gsub(/\\\|/,"#",$0);gsub(/\|/,"\"|\"");gsub(/^\"\|/,"|",$0);gsub(/$/,"\"",$0);gsub("#","|",$0); print $0}'   filename
OR
awk  '{gsub(/\\\|/,"#",$0);gsub(/\|/,"\"|\"");gsub(/^\"\|/,"|",$0);gsub(/$/,"\"",$0);gsub("#","|",$0)} 1'  filename

Output will be as follows.

Code:

|"ABCD"|"EFGH"|"IJKL"|"MNOP"
|"ABCD"|"EF|GH"|"IJKL"|"MNOP"

Thanks,
R. Singh