Four hours in Paris

How long are you staying in Paris, and what does the rest of your trip look like?

There is no best, only what most interests you. So there is no right or wrong answer to your question. I would do a bit of google research and determine which area most interests you:

https://en.parisinfo.com/discovering-paris/walks-in-paris/timeless-paris

https://en.parisinfo.com/discovering-paris/walks-in-paris/montmartre-village-in-paris

This is the only free time on a five day visit

Latin Quarter: Include the fascinating Cluny medieval museum.

I’ve also walked west to the wonderful park of the Jardin du Luxembourg.

Which ever is closer to your starting point, just to reduce time wasted getting there and back.

With just four hours, I would say Latin Quarter. I loved the Jardin de Luxembourg, Sorbonne, Pantheon, Deux Magots cafe (a bit over-touristed but an institution), plus walk to Seine, Notre Dame (or at least the restoration of it).