How long does it take for milk of magnesia (magnesium hydroxide) to work? i haven't gone in 4 days...

A couple of hours: If it has not worked in a couple of hours you should either take another doe or switch to something like miralax (polyethylene glycol) in combination with enemas.

Less than 24 hrs: Usually milk of magnesia (magnesium hydroxide) takes 30 minutes to 6 hours to work, but can be a little longer. It should not take 4 days. I would recommend that you discuss this with your doctor. You may not be drinking enough fluids or you may be too constipated for an oral laxative to work. You can try increasing your fluids and fiber, but also speak to your doctor.