SSO can be validated via SAML(Security Assertion Markup Language) or OIDC(Open ID Connect).

OIDC is built on the flow of OAuth2, so you will see strong similarities with OAuth2 when using SSO(OIDC). But OAuth is very different from SSO because it only caters to authorization, not authentication.

If I have a single set of credentials that allow me to access 4 different platforms, that is SSO. But if these 4 platforms allow social login using google for instance, It is basically the same? As I am able to access the same 4 platforms with my single google credentials.

What you described here is Social SSO, which uses the Google Auth server for SSO. You will see it is very similar to OAuth2 because Google used OIDC, which is built on the flow of OAuth2, as I previously explained.