It is phrasing mathematicians sometimes use. In this sentence 'mod 4' does not indicate an operator but denotes what number system you are in.

For instance the following are equivalent:

2 = 6 mod 4

6 = 2 mod 4

This 'mod 4' can also appear in parentheses or in some other way, but it must appear at the end. Like I said it is not an operator rather it denotes that the entire preceding statement takes place in the appropriate quotient space.

So it is not (multiplying by (4 mod 21)) but ((multiplying by 4) mod 21)