What causes a small hole just below the naval 4 weeks after tummy tuck?

Thank you for sharing your question and I am sorry to hear of this wound development.  Because this is an acute change in your recovery I would reach out to your surgeon to voice your concerns and be seen in person for recommendations on wound care.  Hope this helps.

Dear rsaran,it is hard to tell for sure without an examination. In most cases this type of wounds is caused by poor blood flow or stitches. I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.Daniel Barrett, MD, MHA, MS 
Certified, American Board of Plastic Surgery  
Member, Am. Society of Plastic Surgery

It is not uncommon as stitches dissolve for one to develop wound issues around the navel. Best to see your PS for reassurance or post some photos for a more informed on line opinion. 

Small healing issues can continue for several months after surgery, as absorbable sutures work their way out the skin if they are not absorbing, or if they become mildly irritated or infected. In the absence of other redness, drainage or tenderness suggestive of a deeper infection process, small wounds like this will usually close up with a few weeks of wound care (ointments, perhaps a small ribbon gauze packing, etc.) under your surgeon's supervision.

This opening may be related to a deep suture protruding into or through the skin or could just be any area of decreased blood supply. You should contact your surgeon and have him/her advise you on this. If it is a deep suture, its removal should allow the wound to heal quickly.Steven Turkeltaub, MD

This hole is most likely due to a suture deep to the skin dissolving and working its way to the surface of the skin. Local wound care should help the wound heal in a few days. Best wishes.