As the eldest of 5 children, I say please don't. My mum had a 5th when she was turning 40 because she was a bored SAHM and I really wish she hadn't. Of course I love my sibling but it put a huge amount of strain on my parents marriage, (constant fights all through my teenage years, it was traumatic) my mother's health (she had to have several operations and was in and out of hospital in the years after the birth) and as for me, and the 2nd child, who were 10 and almost 12 when 5th child was born, she basically washed her hands of us, or that's how it felt at the time. She wasn't there for us at all, our home life was completely chaotic and to this day our relationship suffers as she simply doesn't have time to be particularly involved with any of us. She doesn't spend much time with her grandkids either, as there are too many of them and she says she has to keep it even. Which translates to spending very little time with any of them.
That 5th child ended up with drug and alcohol issues, as there was simply not enough resources or attention available to them. Also ended up dropping out of every college course and still doesn't have a proper job.
My advice to you would be to think carefully about the rest of your life, are you prepared to give your already limited time and attention to another person, for another 40 years? Why would you not deepen the relationships you already have with your existing children? Or spend your money making memories with them on holidays. Or possibly get more friends in your life, or a pet.
The other thing that happens is by the time I had children, my mother had only finished raising the youngest sibling, so she was totally over children by then and couldn't be bothered with grandchildren. She wanted to travel alone. Made me so sad as all my friends from smaller families had parents that loved spending time with the grandchildren. She also has very few friends as she was too busy with the family. She never arranged playdates or anything either as she assumed we'd just play with each other which wasn't very healthy.
You say your older 2 are teenagers, teenagers still need a mother. My mother wasn't there for me at all when I was being bullied as a teenager. She was always too busy to ever listen to any of us.
Your family may be different, but I have friends from large families, 5, 6 and 7 kids and most of them harbour some degree of resentment towards their mother for having so many kids.