When you realize you are the fourth child, you are really hoping that the split is two boys and two girls.  Woe betide you if you are a boy and you have three sisters.  It could also be as interesting, notice I didn’t say intolerable) if you are a girl with three brothers.

Hang on, I know someone who was in that exact same situation.  Fortunately, being a girl and the youngest, she could do no wrong in the eyes of her father.

But I digress (as usual)

The meaning of fourth is self-evident, just count to four and it’s the fourth number, perhaps better explained by the fact it is one after the third in a series

Then we use it with other words like,

Fourth-gear, usually reserved for the highway where one expects to geta clear run.  Of course, with more and more cars on the road, sometimes it’s difficult to get out of second.

The fourth estate, no, not what a rich person owns, along with a lot more one guesses, but another name for the press.

One fourth, your share of an estate, if of course, you have three other siblings.  And, in murder mysteries, usually those fourths seem to die mysteriously, and your fourth becomes a third, a half, and then you go to jail.

in fourth place, where it seems all the horse I back run

And,

This is not to be confused with the word forth, which sounds the same but means something entirely different, like

I’m sure we’ve all been told to go forth and be something or other, which means to go forward or come out of hiding

It is also a Scottish river, one notably called the Firth of Forth, and if it sounds odd, so do a lot things in Scotland

You could also place back and forth, much the same as you would in a hospital waiting for the birth of your first child.