G'day Jerry & Mick,

"... All the charabancs mentioned in the threads I found appear to be automobiles (something like the minivan of the 1920s)
..."

Char-à-banc(s) is just French for "a carriage with seat(s)". The Oxford Dictionary defines it as "a long and light vehicle with transverse seats looking forwards" and cites it from 1832.

Presumably, such a light vehicle (filled with day-tripping tourists) could be pulled around level areas with a four-horse team. However, keep in mind that early 20th century motor vehicles were often called by the 'rated horsepower' of their engines ... so that a charabanc with a 4-horsepower engine could well be called a "four-horse charabanc".

Regards,

Bob