What should I do? My incision under my armpit opened up 4 weeks after having brachioplasty.

This is a known problem that can occur with a brachioplasty.  That area is typically the tightest point and has the highest chance of a separation.  This will heal with proper wound care.  Talk with your plastic surgeon to discuss how best to treat this.  In the future a small scar revision may be needed depending on how it heals.
William L. Reno III, MD

Hi Ms. Texas,
Thank you for reaching out with your question regarding the care of your open arm lift (brachioplasty) wound. The first and most crucial step is to contact your plastic surgeon immediately. It's essential that they are informed about your condition, as they will provide specific instructions on how to care for your wound.
Various methods exist to manage open arm lift wounds. These include topical ointments, which can aid in healing. Another option is wet-to-dry dressings, which can help with wound debridement and promote healing. Although a Vacuum-Assisted Closure (VAC) machine, which uses negative pressure to close wounds, is available, it may not be necessary in your case.
Although the wound does not appear infected, your surgeon can advise you on taking prophylactic antibiotics as a precautionary measure.
In conclusion, please make contacting your plastic surgeon your top priority. They are best equipped to guide you through the proper care for your open armlift wound. Additionally, as a cautionary note to other arm lift patients, it's important to avoid excessive movement or activity during the post-operative phase. Too much strain can risk reopening surgical incisions, as in the case of arm lifts or brachioplasty.
Sincerely,
Dr. Katzen

Hello and thank you for posting photos along with your question. First, let me say I definitely recommend that you see your plastic surgeon especially if they are not aware of this yet. Your plastic surgeon can evaluate the wound and make sure its in the best shape to heal. Then they should give you recommendations on wound care. This is a common thing that can happen during brachioplasty and will likely heal with good wound care/dressing changes. It will take time and patience but will likely heal fine. I would limit your arm movement so as not to put extra tension on the open area. I hope this helps and wish you a speedy recovery!Ankur Mehta MD

It is not uncommon for incisions to open after an arm lift. This can even happen a month after the procedure. I absolutely recommend that you see your plastic surgeon to help you take care of it. You may still get an excellent result, but it is important to seek proper care.It is very important that you seek the advice of a surgeon who is experienced with the procedure, and a certified by the American Board of plastic surgery.

I wish you the best, 

Dr. Gabbay

I would see your PS and get some advice on dressings, wound care and subsequent massage etc. This is not uncommon and will heal but best that your PS oversees this for you. 

Great question.  This is not uncommon after brachioplasties.  Some activities can stretch and pull at the incision, leading to a weakness in the area healing from tension.  Your plastic surgeon will definitely want to know and help you heal this area.  It often heals very well with proper care.  Good luck with everything.

Axillary incisions with brachioplasty often have some healing issues.  Yours appears clean and should heal with local wound care and time.  Your plastic surgeon can help you with the proper treatment to expedite the process.

Thank you for submitting your question. Small open areas in brachioplasties can happen. Your wound appears very clean. This will probably be allowed to heal in with dressing changes. Your best course of action is to make an appointment with your plastic surgeon. Local care and close follow-up will lead you to a beautiful result. 
Best of luck!
Dr. K