Imagine you really, really love something. Maybe it's a TV show, or playing with your favorite toy, or spending time with your best friend. Now imagine promising to love that thing forever and always, no matter what. That's kind of like what a fourth vow is!
Except instead of a TV show or toy, the fourth vow is about loving God and helping others. It's a really special promise that some religious people make when they become monks or nuns. Normally, monks and nuns already make three vows when they enter their religious community: poverty, chastity, and obedience. These vows mean they promise to give up any personal possessions, stay celibate, and always obey their superiors.
But some monks and nuns feel called to make an extra special promise to God. That's when they take the fourth vow, which is usually about dedicating their whole lives to helping others in some way. It could be through prayer, teaching, or serving people who are sick or poor. Whatever it is, they promise to do it for the rest of their lives, no matter what challenges come their way.
So in a way, the fourth vow is like promising to love God and others just like you love your favorite toy or friend -- with your whole heart, and forever!