Static common value function in c++

Let's say we have POD Matrix4 class (matrix four by four).

User of this class often wants to initialize variable of Matrix4 type with identity matrix.

There are two choices I know:

1. Provide member function SetAsIdentity for that class that changes the value of *this. So user would do something like this :

   Matrix4 m;
   m.SetAsIdentity();
   
   

2. Provide static member(or non-member) function that returns a static const identity matrix, something like this :

   static Matrix4 Matrix4::Identity()
   {
       static const Matrix4 m({ 1, 0, 0, 0 }, { 0, 1, 0, 0 }, 
                              { 0, 0, 1, 0 }, { 0, 0, 0, 1 });
       return m;
   }
   
   

   and then use it like this:

   Matrix4 m = Matrix4::Identity();
   
   

In my opinion, I'd like to use 2 more than 1 just because it can be used in one line of code. Is there a better way to initialize matrix while making sure Matrix4 is still POD? Could number 2 be implemented with constexpr? Speed difference, if any?