Tightness under bust after tummy tuck. Can doing gentle back bends disturb anything at this point?

I recommend you see the original surgeon for advice on your pain and/or activity level. Your doctor will know best how the surgery was performed, and can likely give you best solution. 

Dear MJM_Happy,every plastic surgeon has his own postoperative protocol which he recommends to his patients. I would suggest you to discuss this with your plastic surgeon.Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

At four weeks post op you are still early in your recovery and a feeling of tightness is not uncommon. I do not suggest you do stretches until you are cleared by your surgeon to do so. I've included a video about walking after a tummy tuck that I hope you find helpful.

Hello dear!!
Thanks for the question and provided information as well.
You are still early on the healing process, it can take up to six months to see your final results, so please be patient. At the stage you are in, discomfort and tightness are very normal. I recommend my patients to wait, at least 4-6 months to resume any workout.
If you have any concerns, please follow up close with your plastic surgeon.
Good luck :)
 

Hi and welcome to our forum!
The procedure of abdominoplasty involves repair of the rectus abdominis muscles which are usually stretched apart during pregnancy. If exercise is begun too early, there is risk of damage to the muscle repair. 
In our practice, we discourage exercise for the first 4-6 weeks, depending upon the status of one's healing. Light exercise is then pursued thru the 8th week, after which time full exercise can be resumed.
Follow your surgeon's instructions as he / she is best able to judge your healing.
Best wishes...