Code:
alias "+FOVSwitch" "bind mwheelup multvar default_fov 80 120 .9;bind mwheeldown multvar default_fov 80 120 1.1"
alias "-FOVSwitch" "bind mwheelup invprev;bind mwheeldown invnext"
bind "mouse4" "+FOVSwitch"
Same script, but smaller and reassigns the wheel to weapon switching. Also, you can change the .9 or 1.1 to the amount you would like to multiply the FOV to instead of all those aliases.