I have 3 kids already and 40 years old. And I am thinking of trying for my 4th. Do you think it's advisable? Will there be any complications?

Having more and more kids in the home is always good. I do not think there should be an issue if you are healthy and mentally prepared to have a child, though there are concerns about how kid's development be after 40, but that too nowadays can be checked and monitored to have everything fine. But, I suggest, you already have 3 biological kids, and why not adopt a kid if you are longing for one. This way, you will have a child and the kid will have a beautiful family as well. Give it a thought, there are many children in this world who need a loving and caring family. :)