From what I heard several years ago, the later 4ESS switches that were installed in the mid to late 90s were of a newer generation than those from 1976 to the late 80s.
And yes, some of these switches are 40 years old!! But as TP said, over time I bet most of the guts have been replaced (though I do wonder about the announcement machines - some of which are quite broken!).
But I think to say relevant, AT&T like a lot of other telcos are going VoIP. I know Verizon basically runs it's old school POTS TDM network and a VoIP network in parallel with gateways to cross connect the two worlds as necessary.
Yeah, AT&T has introduced DMS-250, 5ESS, etc. into the long distance network, but the venerable 4ESS is still the backbone. Just curious what their strategy is to play in a mixed TDM and IP environment moving forward.