The short answer is yes, 4gb RAM can run Fusion 360. However, the better question to ask is how well it will run and for how long.
Fusion 360 is a pretty intense program, especially when working with larger models or assemblies. For this reason, we recommend at least 8gb of RAM if you're planning on using Fusion 360 regularly. That being said, 4gb RAM will likely be enough to get the job done in most cases - you may just experience some slowdown or lag when working with larger files.
If you're only planning on using Fusion 360 occasionally or for smaller projects, then 4gb RAM should suffice. Just keep in mind that you may need to upgrade down the road if you find yourself using the program more often than expected.