As a central topic of quantitative computational chemistry and biophysics, free energy calculation has undergone almost four decades of systematic development. While it has been showing promises, various challenges are also increasingly clear. In the past decade, these challenges have motivated a diverse set of thoughts, assorted developments, and exercises on complex systems. At this critical moment when bottleneck issues are still persistent and various ideas are exerting new hopes, we are gathering major scientists to discuss existing problems, possible solutions, and particularly which directions to follow in the near future on free energy calculations. In this fourth meeting (after the first three meetings in 2013, 2015, and 2019), our theme will be “Underlying Rigor behind Sampling and Energy Functions”. It should prove to be an even more exciting meeting.