my best guess is that it would be some multiple of the wavelength, maybe 1/4 wave or 102"?
maybe a 4' antenna is kinda short for that, unless it's either thin wire, tightly wrapped up until the coil up top, or another multiple like 1/8 wave.
sorry i can't be more specific than that, i've never actually unwrapped a firestik before. i know you probably shouldn't shorten the wire, though.
when we shortened a 7' skipshooter (similar to a firestik) we kept the wire the same length, chopped a foot of the fiberglass out of the middle where it wasn't tightly coiled, and spliced it back together and wrapped it all back up.
worked fine after that, and it was a foot shorter so we could get into parking garages, drive-thrus, etc...
