Here again, we need to rethink our whole concept of work rules. I do think that the four hour shift system can work in this environment. First off, a significant fraction of employees will actually be working two shifts in a row. The heavy shifts are essentially 600 through 1000 and 1500 through 1900. This is where surplus manpower are needed. Yet they naturally dovetail other potential full time jobs. Most retail starts late when the customers are actually at work. And most hospitality work also starts after the supper hour. There is no lack of alternate employment.
Better yet, short shifts can and should earn a premium as well, easily afforded when you lose dead time.
As posted before there is excellent reason to work around a four hour shift environment for many tasks including providing a sustaining shift for the unemployed. This provides an automatic bid for all labor.