Each base-level mergeable item has an item track. You always need four of one of these items to get to the next item in the track.
You can see each item's track in the Merge Bot section of the Collections window:
Once you’ve reached the final version of a mergeable item, that item becomes a fully-fledged set item. So if you had all four of the items, you’d have a pretty exclusive new Merge Bot set.
To reach the final version, it takes a significant amount of the base-level ingredient. Exactly how many depends on how many items are in the track