The unfortunate thing about a 4 hour work day for traditional (read: non-technical) workers is that the advancement of technological tools allows them to accomplish the same amount of work in less time, meaning that if an individual (or entire company) wanted to get ahead, they would still use the entire 8 hours (arbitrary number. Many use even more) to accomplish much more work.

Technology hasn't shortened the amount of work we do, it has raised the standard for everyone to accomplish more.

This is mostly meant to apply to non-technical workers. People who deal with technology have a different and unique set of standards.