Is it normal for my scar to be hard and lumpy?

While it's not the norm, many people will still have a lumpy and firm scar at only 4 weeks after their tummy tuck.In many cases it's simply part of the healing process and can be related to the body's reaction to the absorbable sutures that are utilized in surgery.  I would recommend scar management with either silicone sheeting or a moisturizer with mechanical massage to the area.  It can take up to 3 months in my past experience to improve.  If it seems to be worsening it can be the beginnings of hypertrophic or keloid scarring.  In that case I would see your Plastic Surgeon sooner rather than later.  

It is common for the scar to feel hard and irregular at 4 weeks.  The tissues are still swollen and the underlying sutures are not yet dissolved.  This will soften and irregularity will smooth out over time.  Massage can be of benefit.  Good luck.  Donald R. Nunn MD  Atlanta Plastic Surgeon