hash function for a 64-bit OS/compile, for an object that's really just a 4-byte int

I have a class named Foo that is privately nothing more than 4-byte int. If I return its value as an 8-byte size_t, am I going to be screwing up unordered_map<> or anything else? I could fill all bits with something like return foo + foo << 32;. Would that be better, or would it be worse as all hashes are now multiples of 0x100000001? Or how about return ~foo + foo << 32; which would use all 64 bits and also not have a common factor?

namespace std {
  template<> struct hash<MyNamespace::Foo> {
    typedef size_t result_type;
    typedef MyNamespace::Foo argument_tupe;
    size_t operator() (const MyNamespace::Foo& f ) const { return (size_t) f.u32InternalValue; }
  };
}