On Gmail I get maybe 1 spam a month max in my inbox (and it blocks many per day)
I also have dovecot set to learn Ham every time I file an email from the inbox to a folder for good measure.
1 spam a month would be .018% of emails and 5 x 4 spams a month would be .364% of emails
So I would have gotten about .346% more spam based on the number of emails. In reality, because I don't see all of the mails, it's less. Is a touch more than a third of a percent a 'big difference'? YMMV.