Inline vs constexpr for a static const getter?

In the following piece of code, what function will allow the best optimization for an external use and why ? Is the "Version 4" allowed in C++ 2011 ?

template<unsigned int TDIM> class MyClass 
{
    public:
        static inline unsigned int size()           {return _size;} // Version 1
        static inline const unsigned int size()     {return _size;} // Version 2
        static constexpr unsigned int size()        {return _size;} // Version 3
        static inline constexpr unsigned int size() {return _size;} // Version 4
    protected:
        static const unsigned int _size = TDIM*3;
};

Thank you very much.