You need to learn about system expressions.
The expression left(text,count) will return the first characters of a string (as specified in "count").
To remove characters you can use the replace(text,find,rep) expression. With the example you provided it would look like:
Remember that these expressions need a string to work with. So in case you want to manipulate an integer variable, you'll need to add the str expression to convert it to a string.
If you had a string with the length of 4, you could pick any part of it with yet another expression: mid(text, index, count)
As you can see expressions are pretty vital, so do read up on it. :)