From Xojo Documentation
A 4-byte integer known as a "fourcharcode," is typically used only on OS X or when declaring into QuickTime. If you pass a string of four characters via this data type, it is automatically converted into a four char code Integer. Also, OSType does an implicit type conversion to String when you assign an OSType to a String variable. Note that the String "" is converted to the OSType '????'.
Although this type is available on iOS, it cannot currently be used for anything as you cannot convert it to/from Text.