I cannot tell if your "bump" to your head is in the donor are or recipient area judging by the pic however if you bumped your head in the recipient area and at 4 days, and possibly the sites that were created were large, then yes it is possible that you may have bumped a few grafts out of their site. Or dislodged it enough to bleed but NOT totally lost the grafts. I would suggest that you consult your nearest hair restoration professional for further advice and hopefully in person.
You may seriously damage and cause death of follicles ıf your grafts also pup up through oozing blood. so it is better to look and investigate any grfat seen on skin or hat ??