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?

It is always nice to have more kids in the house. If your current health condition and financially stable, why not? However, the risk of down syndrome will be higher for pregnancy age above 35 years old. You can do genetic test at 10 weeks pregnancy to rule out the DS.