Does this then mean the PS4 is now an unpopular product that Amazon would allow to go out of stock? (This is asides from the fact that Amazon.co.jp would say if it was indeed out of stock).

A retailer is also quite representative of other retailers ceteris paribus i.e. neglecting sales discounts and other unknown promotions which might skew the sales trend or popularity of one retailer over another reflecting customer choices of where they choose to buy their products.

More importantly it is quite obvious there is a shift in buying patterns if a product goes from being consistently out of the top 100 to being in the top 25 while the other goes from being consistently in the top 15 to barely making the top 50.

The question then remains, is this shift temporary or permanent? you can be sure yours truly is on the case and will give you advance notification as the situation unfolds

PS4 is definitely an unpopular product, sales tell us this. Whether it'll remain that way is determined at what games it has for the future.

For this case, all it takes for his statement to be correct is for them to run out for a short while. There's little reason for them to order a lot of stock (because of the low demand and lack of big games coming soon), so I wouldn't be surprised to see small things sold out for a few hours or a day.