I have had the Coke Bottle #4 for quite a while now, I would say more than several years and just this evening decided to take it out to have a play (I still have about 4 more unsolved bottles including a opaque Coke Bottle, another of Wil's designs).
Coke Bottle #4 consist of a regular Coke bottle with a plastic cap, attached to it is a thick chain with a small padlock secured on the end. The padlock is inside the bottle and is restrained inside the bottle by a single chain link over the shackle (see photo). The chain link prevents the padlock from coming out of the mouth and the object is to unlock the padlock and take everything out.
Like most of Wil's bottle designs (and others), they look like impossible objects but we all know that its physically solvable of course; just that it may take a lot of effort and usually for impossible bottle puzzles, a fair (or even great) amount of dexterity. As you can see from the photo, you need the keys to unlock the padlock. I might add at this point that no external tools are allowed as well and you must work with only what you have been given with the puzzle.
I did my usual bit of analysis to try to figure out the best way to unlock and remove the padlock. For a while I was getting no where and I was wondering if I am allowed to remove the set of keys from the chain. I had figured out what to do but couldn't solve the damn thing for a while with the keys still attached. I was rather impatient to get on with the puzzle so I emailed Wil Strijbos, Kevin Sadler and Allard (there's a time difference of about 7 hours between Europe and Singapore). I wasn't sure if either Allard or Kevin had solved the Coke Bottle #4 but figured at least one of them would be able to respond. Surprisingly I got a reply from all three gents within minutes of each other and all confirmed that the keys can be removed from the chain for the solve. Once I did this, I was able to unlock and remove the padlock within minutes; not too difficult I might add. Quite satisfied with my achievement I decided to leave the reassembly until the next evening.
The difficult part came the next evening when I tried to reassemble everything back into the bottle. Like what I gathered from Allard and Kevin, the locking of the padlock back into the bottle was a real pain and caused many puzzlers untold amounts of frustration.
Remember you can't use any "external" implements or tools, just what is provided with the puzzle. I had a fair idea how to re-lock the padlock and went about testing my theory. The re-assembly I must admit is very finicky and requires a great amount of dexterity, but its not something that is excruciatingly difficult to do. Not the kind of difficulty like when you can't solve a high level burr but the type where you know exactly what needs to be done, but you just can't seem to do it physically, given that you can only use what you have that comes with the puzzle. Nonetheless all ended well and I managed to lock the padlock again inside the bottle. Whew!