The different is, if you have 1 U2F key, people who steal your U2F key gain access to one half of the two factors for ALL services you use. With 4 U2F keys, pe

The different is, if you have 1 U2F key, people who steal your U2F key gain access to one half of the two factors for ALL services you use. With 4 U2F keys, people who stole 1 of your U2F keys gain that one factor for only the services that you tied to that keys.