Dealing from the top
A deck of cards is split into 4 piles of 13. You take 1 card at a time from the top of any of the piles. Repeat until there are no cards left. How many different orders could you do that in?
If you crack that, then you should be able to generalise to piles of different sizes, etc.
To be clear, initially there are 4 choices at each step. That remains the case until you empty a pile, and then you’ll only have 3 choices at each step, etc.