I am not you, I am not a doctor, and I am not in your shoes but I will say that if you carry the baby to term, they have to get the baby out somehow. They are not going to say, "Oh, you have already had three cesareans. Sorry, ma'am. You're just going to have to leave the baby in there. Have a safe drive home and enjoy your eternal pregnancy." It is said that it is "not recommended" to have more than three cesareans. This is more about rebutting and re-opening. My aunt has four children. All cesarean-sections. She attempted vaginal with the first child, but labor wouldn't progress, so they cut her. She attempted a V-BAC (Vaginal Birth After Cesarean) with her second. He flatline twice, so she never attempted that again. So, you can relax on that one.
As for HIV, in many cases, with strenuous doctor supervision, cocktails, etc, the virus doesn't pass to the baby. I do know of one girl who was born with it because her mother didn't receive proper prenatal care and used dirty needles for her drug use. The baby survived, but her mother eventually died. [Look up "Wilhemina's War" on YouTube and you will actually see that girl's story, and her grandmother's struggle to take care of her. She lives in my town.]
Just pray to whatever you believe in for guidance. You can abort if you feel that is what is best for your older children, your health, and if you believe [by way of being informed] that your baby will suffer extreme agony. You might also consider that you will be dealing with emotions, such as grief, if you do decide to abort even though you willfully do it for noble reasons, you might still be sad. Just be prepared for that, because you will still be losing a child, even if it is medically necessary. Also, don't think that you're "not allowed" to grieve if you do terminate. It hurts, you're still Momma, and it still your baby that died. So, do cry and mourn. It is still your right.
Nobody with a soul is going to tell you "Yes, abort!"
Hello, sorry for this difficult choice. Really, no one can answer this for you. I'd only say that if your current physician has given this advice to you, it's for a medical reason and therefore, in your best interest. You could step out and see another doctor for a second opinion to confirm if termination is the best route. But you do have three other kids you need to be around for and your best self for. I am a believer in all the many years of training that doctors receive, they are our best source of information and to guide us. Good luck with the decision and it is yours alone. Your body, your life and your decision. Best to you!