Although 4 human strides would be the norm in competition, you would generally build a bit shorter at home so try 4 normal (not large) human strides or even 3 large ones then adjust as necessary.
It should be one horse stride between them which is normally accepted to be 12' or 3.69 mtrs. Always work on this measurement as that is the measure that course builders use when setting a course. If you train at any other distance you could end up with problems at a show.