Which rows to increase in

I am currently making a pair of baby pants and it says:

Inc at each end of 5th row and foll alt (alt-4th-4th-4th-4th) rows

In brackets: alt, middle 4th and end 4th are in bold

Not sure which rows I’m supposed to be increasing in, as I’ve not been knitting for very long

Welcome to KH and to knitting!
This pattern is written for different sizes. You would increase at each end of the 5th row and then at each end of the following alternate rows or the following 4th rows depending on whether you’re making the 1st, 2nd, 3rd, etc size.
The following alt rows would be 7,9,11,13 and so on.
The following 4th rows would be 9,13,17,21 and so on.

Somewhere at the beginning of the pattern is a guide to the different sizes, either the baby’s age or measurements for each of the sizes. Once you know which size you want to make, you can circle or mark that size throughout and follow those instructions.

Thank you so much. That makes sense now❤️