F2 & F4 Intro dates

What are the intro dates of the F2 & F4?  This would be good to know for folks who are contemplating a purchase  of detectors in this price range.  In my case, I REALLY like having four tones.  I would PROBABLY be more inclined to go w/a F4 or an F2(updated w/a DD coil). . . IF I had some idea of how old the circuitry was in these units.  The alternative would be an ACE 350.  I would be giving up the 4th tone and (in the F4) manual ground balance.   any help appreciated.