I can't imagine there are that many 3-4 hour projects that won't already have available code. The most important part of an at home project in my opinion is the walk through.
The best interview I had asked me to go through the project and explain the what and why I did things and asked for high level understanding of what the framework I chose or the browser/node was doing based on what I wrote.
It was an app in React and they went through why I decided to make a component for x, y but not z and then asked if I understood the virtual DOM as a concept.
To me that's the best you can do, you need to understand how the programmer thinks about problems, works through them and that they are engaged in the ecosystem at large.