It has to be the format of x.x.x.x and each x is a 16-bit number, therefore the max AssemblyFileVersion is 65535.65535.65535.65535

It’s a limitation of Windows, not .NET:

http://msdn.microsoft.com/en-gb/library/aa381058.aspx

FILEVERSION: Binary version number for the file. The version consists of two 32-bit integers, defined by four 16-bit integers.