This is what you need (just put this in your config somewhere)
bind mouse4 "vstr gunswitch"
set gunswitch "vstr gun_on"
set gun_on "cg_drawgun 1;echo Drawgun ON; set gunswitch vstr gun_off"
set gun_off "cg_drawgun 0;echo Drawgun OFF; set gunswitch vstr gun_on"
Or something like it
bind 1 ""
bind 2 ""
bind 3 "weaponbank 1;cg_drawgun 0"
bind 4 "weaponbank 2;cg_drawgun 0"
bind 5 "weaponbank 3;cg_drawgun 0"
bind 6 "weaponbank 4;cg_drawgun 0"
bind 7 "weaponbank 5;cg_drawgun 0"
bind 8 "weaponbank 6;cg_drawgun 0"
Try this, but you'll have to replace some values of weapon the weaponbank if you use WASD
bind mouse4 "vstr gunswitch"
set gunswitch "vstr gun_on"
set gun_on "cg_drawgun 1;echo Drawgun ON; set gunswitch vstr gun_off"
set gun_off "cg_drawgun 0;echo Drawgun OFF; set gunswitch vstr gun_on"
edit: you didn't need that first line :P
He obviously wants to be mega creative instead!! :D