If three of your four children were conceived using condoms, I have to ask why is that the only protection being used right now? It doesn't seem like the best plan when it sounds as though none of you are ready yet for her to be pregnant.
There are so many options to prevent pregnancy, and typically condoms are the least effective choice. She might want to consider the Pill or even a Mirena IUD. While there's a slight risk of infertility from Mirena, it is very slight. Doctors give it to women who have had children and may want more. Usually, they won't take that small risk with a woman who hasn't had any kids, but wants them eventually... but Mirena is reversible and unlike the old IUDs has such a low risk that it is often a good choice.
Even just adding a spermicidal insert before intercourse would improve the efficacy of the condoms, and there are no hormones involved. Just don't add that until any oral sex has been finished. I learned about that the hard way, lol.