Some regex basic of your requirements

^ means start
$ means end
[A-Z] means from A to Z in uppercase
[a-z] means from a to z in lowercase
\d or [0-9] means from 0 to 9
{4} means repeat times of the expression (eg. 4 here)

so for your case, this regex should works

/^[A-Z]{4}[0-9]{7}$/

Case insensitive

If both uppercase & lowercase need to be supported, you can write

/^[A-Za-z]{4}[0-9]{7}$/

or add a case insensitive regex flag i

/^[A-Z]{4}[0-9]{7}$/i

Reference

You can read about Regex document here

You can also reach out regex101 to learn more about regex and test it online.