Loosing 4 pounds in a week is not impossible it all depends on what kind of workout you do and also your diet , if your workout is pretty high intensity then yes you can lose upto 4 pounds and no its not unhealthy provided you supply your body with ample macro and micro nutrients basically high protein diet.
Now about your stool , do you have any other symtoms such as vomiting
or abdominal pain
If you have ruled out the above two , just wait and observe for a couple more days and if still the colour is grey then i would reccomend you to consult a physician and do the appropriate tests.
Thank you and take care