Help deciding to have a 4th child or not?

My wife stays home, In turn I work 7 days a week to make up for our family income. My Daughter is 2 and a half, and we have Twins who are 5 months old. The twins were born 3 months premature, leaving them in the hospital for 2 months and at around 7 pounds now at 5 months old. While they are in good health they still need more medical attention then a full term baby, ie perscriptions, doctors appt, hospital visits. We have good insurance and are not left with any medical bills, but the Co pays, perscriptions and etc is wearing us down financially. I am worn out, can't handle triple the crying, (my daughter has been real needy). The Twins are a boy and a girl, leaving us with 2 girls one boy. We plan for my wife to go back to work when they all go to school. My wife worked at a school, and would have summers and same days off. I would like to get clipped and she wants another. I never sleep more then 4 hours usually work 60-70 hour work weeks and try to be an involved parent. I see if we stop a countdown till i can spend more time with my kids and sleep more less work. It's september now so 5 years and counting. But I feel like if we decide not to have more kids I feel like she will resent that. We have both genders. She keeps saying wait 2 years and there will be less stress and I agree but then my countdown to weekends off and a 40 hour work week goes to 7 or maybe 8 years making my oldest daughter 11. That scares me i'm gonna miss out on alot of years. I love kids to me I would love another kid but want to give the kids I have my time. My wife gets all the time with the kids and I feel like why would she not want more kids. I want to live! I'm not going to get snipped unless she says she is done for her reasons. I really am totally done but don't want to ruin her dream family. She also had two rough pregnancys and two C-Sections and wants a normal birth.