Almost 40 and thinking about a baby #4...too late for that???

I’m married for about 8 years, a mother of girl 14, girl 4 and boy 3, finishing college and finally getting ready to re-enter work force. I had my last baby at 36. But suddenly I feel like being pregnant again, hold a newborn...I would love a second baby boy. Is that crazy to think about more babies when I’m 39 already?

Please, give me your real thoughts on this! TIA