Last year, Samsung let us customize the hinge cover for the Galaxy Z Fold 2 and it has now extended the idea with the Galaxy Z Flip 3 Bespoke Edition. The Galaxy Watch 4 is joining in as well, but not the Z Fold 3 unfortunately.
Right now, the Bespoke Studio only serves in markets including the US, the UK, Canada, Australia, France, and South Korea. Samsung plans to expand this list eventually. Note that the pricing for all Bespoke models is the same as the original devices. The Bespoke Edition phones and watches will also come in a slightly different case.
As for the Galaxy Watch 4, you can pick between several strap designs and colors for all the watch sizes, but the watch faces will remain the same.