There would not be any shortage of workers. Let’s analyze this. When unemployment rate is calculated, there are two questions that are asked. The first one is, “Are you employed?” If the answer is yes, then the next question is not asked, and you count as employed. If you answer No, then the next question is “Were you actively searching for jobs” If the answer is yes, then you count as unemployed. If your answer is no, then you are not included in the Labor Force.
Having said that, this means that people who have given up searching for jobs are not considered unemployed, and people who would like full time employment but can only find part time, are considered employed. There is a huge gap in hours wanted for work than what it is actually being worked. Many people would like more hours of work or at least “a” job, but that is not measured in the monthly unemployment rate we all see in the news.
We could also think about the amount of stay-at-home parents who decide to stay home and take care of the children. Reducing the work-day and increasing the availability of jobs would make it so families could have 2 incomes. For example, the husband could go to work from 8am to 12pm while the wife stays at home. When he comes back at 12:30pm, the wife could go to work from 1pm to 4pm and the husband will not stay at home until the wife returns. This dynamic will not only put the family in a better financial position due to the two incomes, but also the load of responsibilities about home care and child bearing would be shared between both parents.
We could include college students in this example, who could find Full-Time employment while still pursuing their education. They could not only earn an income during that time, but they could also pay for their education, releasing the burden parents have. They could even avoid student loans, which many people think is the next bubble to explode. We could get rid of that idea that college students are poor.
But there is more availability of workers. People could retire at an older age if the work-day is only 4 hours. Perhaps 70 or 75 would become the new standard. With short work-hours, people could choose to remain in the labor market for much longer time.
And finally, a source of workers that is inevitable. If work-hours are reduced, firms will react to it to try to avoid the increase in labor costs. They will deal with the same way they deal with demands for better wages or better working conditions. They will react by laying workers off, or perhaps relocating to another city or country with cheaper labor, or automating the production processes. A concrete example can be seen in an article from the Wall St. Journal entitled Minimum Wage Backfires. McDonald’s moves to automate orders to reduce worker costs. In it, the author explains that Mcdonald’s would react to minimum wage demands by replacing workers with automated kiosks that would allow customers to place and customize their orders themselves using a touch screen or even their cellphones. Normally, this would be a problem, but since there will be more availability of jobs, these people would be able to find employment somewhere else.
In conclusion, there is enough labor force around to find the equilibrium between supply and demand for labor.