Men maybe you can explain?

My bf and i have been together a long time getting married finally. We have 4 kids together all biologically ours 3 boys/1 girl ages 13,8,6 and 4.To get to the point we are ages 33 and 34 i feel that 4 kids is enough but he feels maybe we should have one more!?He has mentioned more than once that he would love to add another son to the clan in particular or a little girl for our daughter who is 8 yrs old to bond with.I am a real estate agent(just went back to college)who makes a good income and my man is a speacialized mechanic but we live in 3 bedroom 3 bath home which is comfortable also not big enough as is. The taxes in our town are high up to 10 grand now on top of our morgage and private school tuition.So money is a bit tight and i think we should be more concerned with figuring out a way to help our eldest son pay for college since thats only 4 yrs away?I dont understand why he longs for another child?