I'm not sure what version we're on, this would probably be 4.0.
Internally, the mysql database schema was called "tumblr3" as an artifact from that. Or at least that's the explanation I heard when joining the company in 2010.
I don't think anything was ever versioned after v3. So that means if you're saying this is now 4.0, I guess we should expect the biggest change in 16 years :)