After orchestras, military bands, swing bands and big bands, popular music devolved into a group of usually 4 performers with guitars, a drummer and a vocalist. Which was the first such 'band'?

This is a difficult question to answer for sure. The first limiting factor to your question is the presence of a guitar in the band. The guitar did not really have a place in the big band because it was not a loud enough instrument. Les Paul and Leo Fender changed the use of the guitar by electrifying it making it louder. The popular music at the time to first really utilize the electric guitar were both Jazz and Rhythm and Blues.

So possibly the first band to have the structure we have come to know could have been Harlem Hamfats. They had six musicians including the four asked in the question along with a piano player and a clarinet player. Their first album "Oh! Red" was recorded in 1938.

Despite their name, the Hamfats were based in Chicago.[4] They were assembled by record producer and entrepreneur J. Mayo Williams for the purpose of making records - perhaps the first group to be so created. None of the members of the band were actually from New York. Kansas Joe McCoy (guitar, vocals) and his brother Papa Charlie McCoy (guitar, mandolin) were from Mississippi; Herb Morand (trumpet, vocals), John Lindsay (bass), and Odell Rand (clarinet; 1905 - 22 June 1960) were from New Orleans; Horace Malcolm (piano), Freddie Flynn (drums) and Pearlis Williams (drums) were from Chicago.

https://en.wikipedia.org/wiki/Harlem_Hamfats