Why is there a new bump on the bridge of my nose four months after a septoplasty/turbinate reduction? (photo)

Followup with your doctor about your concerns.The nose
can be swollen for up to a year or more. Swelling will depend upon the
nature of maneuvers employed, presence of grafts or implants, thickness
of skin, open vs closed, revision vs primary, etc. Kenneth Hughes, MD Los Angeles, CA

I agree that it does take several months for swelling to improve after nasal surgery. The bump may be from an unaffected nasal bone pyramid which is more apparent now after some healing has taken place from septoplasty. I would speak with your surgeon about your concerns.

I inform my rhinoplasty patients that it will take 3-6 months, if not longer, for complete healing.  Sometimes such irregularities do indeed improve with time.  Having said this, it is possible that what you consider a bump is something that was there before, but more apparent after surgery.  Try to be patient but speak to your doctor after about 6 month about it.  Please try to wait at least 1 year before considering revisions surgery.