Do you mean a 3 piece plus a crooner, or do you mean 4 piece as in guitar, bass , drums, and ...some other instrument ?
I was in a 3 piece ( git , bass, drums ) for a few years when i lived in Northampton, and we picked our tracks so that the sort of thin sound you mention when solos
kicked in was partly avoided. The other ways is your guitarists sound. Our git was well aware of this thinning out, and set his tone accordingly.
Being a grade 8 teacher, he knew what to do to fill the sound, rather than leave that to the bass, which is much harder to do