Joseph says that the product of a four digit number and a 1 digit number is always a four digit number does Joe didn't make sense

The answer is no. Not always.

There are times that the 4 digit number will involve carrying and it will be resulted into 5 digit number.

For example:

9 999 x 9, where 9 999 is a 4 digit number and 9 is a 1 digit number.

When you multiply this one, the answer will be 5 digit number.

9 999

x 9

=> 89 991, thus Joseph's statement does not apply to all multiplication process because not all 4 digit number multiplied with 1 digit number will result to 4 digit.