Assuming a basic CSV — no commas or line breaks inside quoted fields, this will pick up the 4 fields, duplicate one of them, and insert constants:
^([^,]+),([^,]+),([^,]+),([^,]+)$
^ anchor to start of line
( start a capture buffer, the first capture buffer is referenced with \1, etc.
[ start a character class
^, search for anything which isn’t a comma
] finish the character class
+ one or more occurrences (use * if you might have empty fields)
) finish the capture buffer
, look for a comma
…
$ anchor to end of line
Replace:
\1,\1,xxx,\2,\3,yyy,zzz,\4
You could also import the CSV into a spreadsheet, do the manipulation, and export as CSV.
Cheers