Due to new regulations, the classic Sn-37Pb (wt.%) solder must be replaced by lead-free material. There are many alloys that could be used instead of this classic lead solder, including quaternary Ag-Cu-In-Sn alloy. The CALPHAD method was used for thermodynamic description of this quaternary system. Good agreement between calculation and available experimental information was found. Solidification of the promising lead-free solder Sn-1.5Ag-0.7Cu-9.5In (wt.%) was performed using the Scheil approach, and good agreement between this calculation and differential thermal analysis results was found. The obtained set of Gibbs energy functions can be used in the future for expanding the quaternary system to high-order ones.