No problem! If you’re just using UE4 as a hobbyist, or only planning to use Blueprints, you’ll probably be fine download UE4 onto your D-drive. The only slowness you’d notice would be loading up the project.
However, if you plan on using C++, or are working with UE4 professionally, I would definitely invest in an SSD (I’d recommend at least 256 GB. I personally bought a 512 GB SSD, but I use UE4 for work).