I am experimenting with elevated mounting of a monoband 80m vertical right now... I've been doing a lot of research and learning. Let me just share some of the findings I've made.
Mounted on the ground... more is better, concentrate on more close in rather than a few long radials.
BUT, elevated mounting... 1. Insulate the feedpoint from ground 2. put a good RF choke on the feedline, else the antenna system will see the coax shield as another 'radial' 3. the radials and/or counterpoise must be TUNED... But that is ELEVATED radials/counterpoise remember... you fence is not elevated, only the mount.
And your fence is not likely to be tuned, nor can you tune it, since it is in contact with the ground. Even if you did put up the proper wavelength of fence, the ground would couple to it and detune it anyway... The purpose of the ground or counterpoise is to shield the vertical from ground losses (get between the vertical and the earth ground and keep the ground from sucking up the radiated energy) and provide an efficient path for return currents (allow the 'other half' of the radio wave current to make it's way back to the feedline).
With all that said... experimentation can't hurt. If you plan on working 40 or 80m, I would definitely go with the 75' piece of fence... the 15' piece is likely too short and before it is suggested, adding radials to the fence is not going to work.. not if the vertical is 4 feet off the ground. Try mounting it temporarily and see if it tunes well... THEN see if it plays well on the air. Just because it tunes, does not mean it is efficient! A dummy load tunes well...
Also, it's not going to be omnidirectional... just like when you mount a whip on a car, the metal mass of the car affects directionality. You'll likely have signals favoring the directions that the fence extends away from the antenna.
Basically if you are looking to ragchew with friends in another state, you might get lucky with something that works. If you are looking to work a lot of DX, I'd forget the fence. Either ground mount it with 20 or so radials on the ground or mount it as high as possible and use 4-8 tuned radials at the lowest frequency you want to operate sloping down at a 45-50 degree angle. (The slope brings the feedpoint impedance up to 50 ohms. A dipole is 72 ohms, an inverted vee is close to 50 ohms, a vertical over poor to average ground is 40-60 ohms, a vertical over perfect ground is about 36 ohms, but an elevated vertical with radials running straight out horizontal is only 26 ohms!)
I am betting that like me, through trial and error, you will come to the conclusion that anything MAY work, but only certain specific methods will work WELL.