Fourth Hat of Antique Hatrack Collection
Looking at the West of Loathing Wiki article on the hatrack provides some insight as to what it could be; it seems the hats match the order yours are in.
By process of elimination I believe the missing hat to be the 'really nice cowboy hat' which is found in the Temporal Nexus at the West Pole.
For reference, here is the hat list (information from the article); they must be in your inventory at the end of the game for them to appear in the credits sequence:
- four-gallon hat, barely-enchanted hat, or floppy derby - picked up at Boring Springs, dependent on the player's class
- bear hat, cat hat, or owl hat - a gift from Rufus after the first couple of postcards, if you picked a name at character creation by shooting the stuffed bear, cat, or owl figure
- fake Pope hat - acquired from Saint Pope in the Old Mission catacombs
- really nice cowboy hat - found in the Temporal Nexus at the West Pole
- prototype Stetson - in the office at the Old Millinery, in a locked safe behind the locked door
- turnip crown - the dusty turnip from Boring Springs, plated at The Silver Plater for 5000 meat (normal) or 20000 (hard mode)
- gold tooth crown - made by the bone-obsessed hippy in Fort of Darkness from 23 gold teeth
- El Vibrato crown - reward for activating the El Vibrato "Roberto Containment" machine in Deepest Delve Mine
- yeast-covered hat - lying on the floor in Ol' Schmaltz Brewery
- strange head sack - for trading an old patrol cap to the second hippy at Lazy A-Dude Ranch
- spittoon hat - from the spittoon in Clooncy's Bar in Frisco