There are differences, many of them are included in the 4th edition optionals but some such as leaders for AFVs and other units being the same are significant.
You need to think of Modern PG, PG, Infantry Attacks, and American Civil War PG (remember that one? a sample game was available on this site, and its a back burner project at the moment) as separate but parallel rules tracks.
The designer of the ACW game with Richard Dengel (sp?), the designer of Huzzah! and Rebel Yell! Not sure if he's still interessted as the Huzzah! system seems to be doing well.