Cool, the key is in the middle if statement. Originaly it was:
if ((strpos((",".ValidatedField("Registration","Registration").","), "," . "3" . ",") !== false || "3" == "")) {
To add the or to it:
if ((strpos((",".ValidatedField("Registration","Registration").","), "," . "3" . ",") !== false || "3" == "") || (strpos((",".ValidatedField("Registration","Registration").","), "," . "4" . ",") !== false || "4" == "")) {