You're correct - order matters, so it would be 10,000 for 4 digits, 1000 for 3 digits.

My original numbers were based on order not mattering...

https://www.statskingdom.com/combinations-calculator.html

If someone wants to figure out the last 4 digits of your SSN randomly.

All 4 digits unknown = 10,000 combinations

Last digit known, 3 digits unknown = 1000 combinations

The last 4 digits used to be assigned in order - so if they know your birthdate they could make some assumptions about the beginning numbers as well.

(*edited to correct numbers...)