To get the v3 from v4 he made the struct like this:

struct v4
{
  struct 
  {
    union
    {
      v3 rgb;
      struct 
      {
        real32 r, g, b;
      };
    };
    real32 a;
  }
}

But why can't we do it like the v3 and v2.

struct v4
{
  union {
    struct {
      real32 x, y, z, w;
    };
    struct {
      real32 r, g, b, a;
    };
    struct {
      v3 rgb;
      real32 ignored0_;
    };
    real32 E[4];
  };
}

I tried this and nothing seems to be obviously broken. Not sure why he is making it like this in case of v4.

You can also skip the top level struct and removed the ignored members:

union v4
{
    struct {
      real32 x, y, z, w;
    };
    struct {
      real32 r, g, b, a;
    };
    v3 rgb;
    real32 E[4];
};

i see, so there is no need to make the whole memory reachable in each thing that is unionized