Oh I didn't realise that, that seems unfair to me to have the same operating system on the 1st watches, then something completely different for the watch 4.
Samsung changed the operating system for the watch 4. This allowed users to have greater access to Apps from Google Play Store, whereas the earlier watches were and are fixed to Samsung Tizen Apps in the main. The same with the watch faces.
I updated from the Watch 2 to the 4. A year ago. I rather think that they will be bringing Watch 5 out later this year. But I can certainly recommend the 4 series.