I don't think your assumption is correct. Symphonic music was expensive to produce and was the province of royalty and wealth. It did not lead to pop music. Folk music leads to pop music.

Parallel to this would have been folk music, including Gypsies, family musicians, tavern groups (think Irish pub music to find your 4 piece ensemble) and tribal ritual music. This would have been the 'pop' music of its time, as well as the tradition that held groups together, and the accompaniment to celebrations, and the news (traveling minstrels singing about the goings on from other lands).

The most important influence on the ensemble you mention, drums, bass, guitar(s) and singer(s) was the blues, not classical music. You'll find that string orchestra accompaniment on rock and pop songs did not occur until a band had the cache and the cash to afford it.

Fingerstyle rock guitar (Fleetwood Mac's Lindsey Buckingham) grew from Bluegrass traditions adapted to rock sensibilities. The Eagles grew from the tension in the back between the country music artists and the rock artists. Iron Butterfly was, if you believe the liner notes, classically trained musicians playing acid rock.

If you know some Bach two part inventions, or some Brahms, or such, listen to Emerson, Lake and Palmer to hear the parallels in their rock music. For a listen to pure pop with classical arrangement, try Haven't got time for the Pain by Carly Simon. Folk rock? Stairway to Heaven by Led Zeppelin; lots of repetition of themes, with variations in orchestration to build tension, just like classical compositions.