John Gill Commentary Joshua 21:39

"Heshbon with its suburbs, Jazer with its suburbs; four cities in all." — Joshua 21:39 (ASV)

Heshbon with her suburbs
Which was formerly the royal city of Sihon king of the Amorites, and was first given to and rebuilt by the Reubenites, but was upon the border of the tribe of Gad; and appears from hence to have been one of their cities, (Numbers 21:26) (32:37) (Joshua 13:17Joshua 13:21Joshua 13:26) ;

Jazer with her suburbs ;
sometimes called Jaazer, another city in the same country, and near Heshbon; see (Numbers 21:32) ;

four cities in all :
which were Ramothgilead, Mahanaim, Heshbon, and Jazer.