4 weeks into tummy tuck, new vertical mark / scar - what is it?

It's difficult to tell from the photo but it may be an indentation from the seams of the binder or perhaps a garment that folded underneath the binder. If so it should improve as you take a break from the compression. If you have a t-shirt or other clothing between your skin and the binder be sure that the material lays flat against your skin.

The vertical mark that you mention is due to compression from a seam area of the binder. This will go as the swelling subsides. It will take about 3 to 6 months for the swelling to settle down and will continue to settle for almost 12 months. 

The vertical mark could be the result of a compression garment that is too tight.  Our office provides our TT/lipo patients with an adjustable compression garment.  The  patients like the support it provides to the back and to the abdomen.  The only complaint I have heard is related to the "itchiness" of the fabric which can be remedied by wearing a soft t-shirt under the garment.Any concerns of the compression garment being too tight or not fitting properly should be addressed in person with the plastic surgeon.  The compression garment could cause a burn or pressure necrosis if too tight.