Windows Server 2003 is built on the same architecture as Win XP. Both do not support .net 4.5 and both are past end of life support.
SambaPOS V4 now requires .NET 4.5 as 4.0 is too depreciated to use. The decision was made a few months ago to abandon Win XP and .NET 4 support as it restricted programming and was holding SambaPOS back.