Hard painful lump in breast 4 months after breast reduction. Is this common?

Its best to return to your surgeon for review.  There may be several causes including scar tissue and fat necrosis and you need an in person evaluation.

Congratulations on your surgery and I am sorry to hear that you are concerned.
I would go to have that lump checked, following which your surgeon might decide to investigate it, scan it for example.
It can be scar tissue- gentle massage would help, can be fat necrosis- very common in reduced breasts, or some other pathology, although less likely, has to be ruled out.
Take care!

Diagnosis of the cause would be best made by the doctor who performed your surgery. It may be a lump of scar tissue from the old infection, or it may be a specific type of hardening of some of the fat in the breasts, called fat necrosis, that is common after reduction surgery. You'll want to see your surgeon to make sure that it is being followed appropriately. 

I highly recommend that you return to your surgeon for evaluation of the lump.  It could represent one of several things.  Most of all you want to make sure that this area is healing properly and no other issues are involved.