My 4 month old ate a sock about 3 days ago (I was out of town my mother was puppy sitting) she didn't let me know until 2 days ago. Took him to the vet they did an x-ray, the vet said he should be fine to just observe him, and that he should pass the sock soon. Still very worried though. He seems fine, eating, playing, potty is normal, no abdominal pain.

I'm presuming that you're checking his stool for  'foreign objects'.  By four days, I'd think you should have seen some evidence of the sock.  However, if it's a natural fiber it may have been digested - dogs digestive juices are really rough.  If it was an artificial fabric, it may not have been digested and may still be floating around in his stomach.

Now, the problem with an X-ray is that you're looking for a soft object (a sock) in a mass of soft objects (his organs), it may be in there and simply not be visible.  An ultrasound, I think, might be better at finding a sock than an x-ray.

In any case, since you said he seems fine, I'd simply watch him (and his stools) for the next week or so.  If he passes it all well and good - just don't leave socks where he can get to them - for some reason, dogs love socks.  If he "goes off" before there's an indication that he's passed it, it's time for an emergency vet visit - my opinion, others may differ and that's what we're here for.  Some have other experiences that might feel more appropriate ....

It can take socks a long time to pass. Like a couple weeks depending on the sock and size of dog. As long as he's eating, not vomiting, and still having normal bowel movements I would just watch, as Al has said. I would check poop for at least a couple weeks before I would rule it completely digested if it were me.

As said above observe and wait. Watch the feed intake poop and pee outtake!