Casey-
I figured it out!! So I set the settings as Alena instructed. Still didn't work. I looked at some different code elsewhere. I saw that in similar lines of code no one put "to[]": It was always "to" : [ ]
So I changed
sendgrid_mailsend.execute({data: {"to[]": [email1, email2, email3, email4]} });
to
sendgrid_mailsend.execute({data: {"to": [email1, email2, email3, email4]} });
Very subtle difference but I think it might have been creating an array within an array? Not sure. But I can confirm that I have successfully received the emails at every email address that I wanted.
Cheers,
Nathan