Joe says that the product of a 4 digit number and a 1 digit number is always a 4 digit number does Joe's statement makes sense?

No, what Joe said isn't necessarily true. Though it is common that the product will be a 4 digit number, because you can use any number, like a large 4 digit number like 9999 or a large 1 digit number like 9, that isn't true. For instance: "9999 x 9" is 89991 which is 5 digits instead of 4 because "9999" is the biggest 4 digit number and "9" is the biggest "1" digit number.