I actually had access to internet over a cat5 ethernet cable a bit longer than 400 ft and it worked all right.

There are two limiting factors for the distance. Signal strength is a soft limit – it is guaranteed at 100 m (333 ft), so there has to be some reserve and as a result the signal is still usable at 150 m (500 ft). The other is collision detection – but that's not a problem if the link is operated full-duplex.

So if you make sure both ends are connected to either computer or proper switch, it should be workable at 400 ft. Also this cable I was using was lying on the roof and even hang from one roof to another and once again over ~20 m gaps (attached to support wire, but that means every meter or so) for at least two years and survived that, so having cable lying outside on the ground should not be a problem either.

–Jan Hudec