What is the difference in these two tenses?

Hi E.S.,

It depends on the situation -

If you have been living in Paris for four years and it is still going on, you will use the present tense + depuis:

J'habite Paris depuis 4 ans.

If however, you don't live there anymore but you lived there for four years you will use the perfect tense + pendant and say :

J'ai habité Paris pendant 4 ans.

Take a look at the following Kwiziq lesson for more examples -

https://french.kwiziq.com/revision/grammar/expressing-for-duration-with-either-pendant-durant-depuis-or-pour

Hope this helps!