However, that's a wildly simplistic view of things.
You can't just say this without realizing that there are severe drawbacks to this. We can't just all work 4h instead of 8h and expect to maintain our standards of living, all the benefits of our times, maintain scientific progress, maintain national security and many, many others.
The ultimate goal should maybe be productivity, and if 4h is better than 8h for whatever reason, then let's do that. But it's definitely not so simple.