You will need a fan if you're regularly using the Pi for more extended periods. Regardless of what tasks you perform with the Raspberry Pi 4 or for how long you're usually using it; it is still best to install a fan considering the tiny board's upgraded specs.

Does my Raspberry Pi need a fan?

The Pi 4 needs a fan

A heatsink installed inside the Pi 4's official case will do precious little to avoid throttling the CPU (and likely other components, as they all get very hot).

Does the Raspberry Pi 4 overheat?

The Raspberry Pi 4 is a magnificent single-board computer that offers a lot more than its predecessors in terms of performance. ... If you have tested a Pi 4 for any time, you might have noticed that it can run quite warm. Consistent overheating can seriously damage the board.