I've had 3 C-sections and all went well,but during my last one,my thinned uterus (thinned due to the sections) got stuck to the bladder and they had to cut the surface of the bladder to separate them.I had no extra pain due to that and recovered quickly.
I had my 5th c-section April '08 and the doctor said nothing to me or my husband about having another c-section if we wanted to get pregnnt again. I was 44 (almost 45) at the time of my section as well. I think you'll get a wide range of opinions on this matter if indeed that's your concern. The person best to answer your questions is your doctor as he knows or has your wife's medical history, etc.
My sister in law had 7 c-sections. Her oldest is 32 & youngest is 16. I think it really depends on how you heal.
What exactly is your concern? That she is having a 4th cesarean section?? I know of one woman who's had 4. I think 4 is pushing it, personally I don't think it's problematic at 3 - C-Sections, but it just depends on how you're uterus is doing and/or has healed. She should discuss this with her doctor if you concern is the number of c-sections she's had. But I'm not sure really what you're concern is.
I am going to be having my 3rd c-section in 3 years :) I asked my dr. and he said he looks at the uterus and would advise me if having another child is not an option. I think that your sister will be fine, congrats on being an Aunt again!