There's "quality of defence" as in how good the team is at not conceding goals. That's not just about the back four. And there's "quality of defence" as in what the players in the back four contribute to the team. That's not just about how many goals you concede. Important to be clear about which of the two we're talking about.
All things considered, I think it's fair to say according to the latter of those of definitions that we now have a back four that is of comparable quality to City and Liverpool. All 3 teams have world class quality in 3 of those 4 slots, and a bit less in the 4th. And they are all back fours capable of contributing quite a lot in transition and attack. Liverpool's more than the others, but then that comes out with a correspondingly greater defensive onus on the central midfield that contributes less to the attack than ours and City's does.
Tough comparison with Chelsea, as they play a back 3 (or 5). No one else should be close. It's really looking quite markedly like a clear top 4 this year, much more so than a year ago.
By the former definition, that'll depend a lot on selections, posture and performance in other positions too (especially in goal and in the midfield), and on how we do on defensive set-pieces (which again involves a lot more players than just the back 4).