400 yards is pretty damned far for small game. Do you live on the prairie?
I'm wondering, because often people want far more than they need, and then get upset because it's far more than they want to pay for. It's also a helluva lot different with a number on a page than it is in real life. I don't want to be condescending but try dropping your gf'd on the sidewalk on a level stretch of road, driving 400 meters on the odometer, and then stop, get out and wave at the small blob in the distance.
I tried this at 250, 500, and 1000 yards last year and was suprised at what it looks like over iron sights (I used my crosman 760 airgun and also tried it with my 4x scope-handheld). I figured out that even at "only" 500 meters, I'd require a small miracle to hit a man (woman) sized target, standing offhand.
Prairie dogs, coyotes, and other small game at 400 yards sounds like varminting to me, and it's not a cheap+easy pursuit. It's also far beyond the capability of a .17 caliber rimfire.
A .223 can do what you are asking (small game at 400 yards), but you better be a
damn good shot, use a very good rifle and scope combination, handload carefully with quality bullets after developing a good load, and be a range and wind estimating wizard.
P.S. Don't forget your eye protection
Good luck,
Bobby.