From the GNU awk manual (http://www.gnu.org/software/gawk/manual/gawk.html#Splitting-By-Content):
$ awk -vFPAT='([^,]*)|("[^"]+")' -vOFS=, '{print $1,$4}' file
"[email protected],www.example.com",field4
"[email protected]",field4
and see What’s the most robust way to efficiently parse CSV using awk? for more generally parsing CSVs that include newlines, etc. within fields.