The plan would allow workers to spend fewer hours in the office - with no reduction in pay.
The test run has been proposed by the left-wing party Más País - which argues that longer hours don't necessarily mean higher productivity.
Details of the project have not been thrashed out but the party has proposed a three-year, €50m, project that would allow interested companies to join the trial with minimal risk.
Companies' costs would be covered at 100% the first year, 50% the second year and 33% in the third year.
Iñigo Errejón of Más País said on Twitter that the four day week was launching the debate of our times, adding that its time had come.