I use a s&b 6 x 42 fixed at rabbits a 50yards with my rimfire........i would not go any lower then x6 myself.
I do use a 1-5 x 20 simmons on my .22 air rifle at times but only really for woodlands and short range shots, 10yards -40yards (not at night) perfer a little more mag as i only go for head shots, and x4 can be a little limiting on air rifle at 50 yards!!
rimfire is a little more forgiving at that range.