When a horse gallops..

[Vics128]

Is there ever a point when all four of its legs are off the ground?

[carmalee]

yes

[heathfield]

Check it out here.

[lankeela]

Actually there is a moment of suspension (i.e. all four feet off the ground) in a trot, canter and a gallop. Only when walking does a horse have at least one foot on the ground at all times.