You are getting 3 to 4 miles of range through heavy brush with the 6320? Not to doubt your word but you must have received a "suped up" model as in most cases with FRS/GMRS radios you won't get more than a mile (if you are lucky) in those conditions.

I cannot think of a single FRS or GMRS radio that will transmit 3-4 miles through brush, trees, hills, etc.. It is because of the nature of the radiowave that those types of radios utilize. They do not work well in those types of conditions.

You might...and I stress "might"... be able to pull that off with a 4 watt GMRS unit with a 6 or 8 inch whippee antenna. Pryme radio products makes one of these and the cost is about $190 a radio but it does come with a heavy duty rechargeable NIMH battery.

For more info on this type of question I would suggest visiting.... www.popularwireless.com .

Sorry I could not help more.