My 6 pound Yorkie chewed and ate a spare rib bone 4 hours ago. Is he going to be OK?

Was it cooked?
Every dog seems to be different. Some can swallow bones and be fine, others have a hard time with it and can end up at the vets.
First thing you should do is feed your dog cottonballs. Tear them up into pieces and feed them in your dogs food, or cover them in something that he will want to eat. For his size, you'll want to feed him two. These will help if the bone has splintered (which is common in cooked bones), they should wrap around the sharp edges and hopefully help them pass.

Look for things that may indicate the bone has splintered or is impacting somewhere, such as blood in the stool, having a hard time going to the bathroom, crying, hunched appearance, not eating, or when he eats he vomits shortly after, really anything out of the ordinary. If you notice anything, your dog needs to go to the vet to get an x-ray done. While a lot of dogs are fine, there is the odd dog who has problems and needs the bone removed surgically.

Good luck.

A spare rib bone is pretty large for a 6 lb Yorkie, even if it was a small pig. Are you absolutely sure he ate the whole bone, and didn't just hide it somewhere? Finding it after you have run to the vets and had x-rays done that didn't find anything is both embarrassing and expensive.

Even 4 hours is enough time for him to decide that "he doesn't feel quite so good any more". If he did get it down, (I suspect the bone is at least 3 inches long, or half a dollar bill), it will be pushing on his stomach and would expect to cause him to be nauseous. Unless he gets it lined up just right, throwing it back up is problematic. Uncooked bones are easier to soften with stomach acids, but it takes a day or so.

I'm not sure I would want to wait a few days to see if things will come through OK, or not. It may be possible to retrieve it without surgery, while it's still in the stomach, if your vet is equipped with some of the expensive endoscopic 'goodies'.