Hello Rohit

If there are different cases, we add them.

If there is one case, we multiply them.

For example:

If we need to find 4 cards of the same suit, 

4 cards can be from Diamond, Spade, Club or Heart.

So, either we can take 4 cards from Diamond

or

4 cards from Spade

or

4 cards from Clube

or

4 cards from Heart 

So, in this case, the combinations get added.

Now,

If we need to 4 cards, where 4 cards belong to 4 different suits.

Then, we choose 1 card from Heart

AND

1 card from Diamond

AND

1 card from Club

AND

1 card from Spade.

These, 4 cards make up 1 case.

So, we multiply them

Did you check out the video here ?

https://www.teachoo.com/2400/609/Example-19---What-is-number-of-ways-of-choosing-4-cards-from/category/Examples/