The PS4 is not yet retro, it’s just been re-categorised from ‘current gen’ to ‘last gen’, as the PS5 has taken its place as the ‘current gen’.

When the PS6 eventually launches some time around ~2030, it would still be ‘next gen’ for a while until it reaches mass-market adoption. At that (largely arbitrary) point, the PS6 would become ‘current gen’, the PS5 would become ‘last gen’ and the PS4 would go on to be considered ‘retro’.

Just for reference - by that point in time, the PS4 would likewise also be ~18 years old.