I think four hours is good notice, all ssorts of things happen to all of us, judging by some reports on here the rude WG's just turn their phones off of don't answer the door.
Depends on circumstances whether notice seems adequate. If the meeting has been arranged for a while, and you confirmed recently that all was going ahead, a same day cancellation can feel like not enough notice. I would also take into account the length of the booking.
But in the end, the thing I always ask myself is not so much whether she gave enough notice with the cancellation, but whether I feel massively let down - which also involves things like whether she offered a credible reason, whether she told you over the phone, by text or just did not bother, and whether she offered an alternative date / time straight away.
