Shiba and Shikoku - NOT the Same Breed in Different Sizes
The popularity of shibas has risen drastically over the last two decades so it's not surprising that people interested in the native Japanese breeds often wind up bringing one home before they're even aware that other native breeds exist - with the exception of the akita. Yet once they do find the other medium nihon ken, whether it's the way they look or the basic (re: generic) description of their temperament, or possibly a combination of both, they seem to gravitate toward shikoku under the mistaken assumption that they're more or less the same dog in different sizes. But it must be said - SHIKOKU ARE NOT JUST LARGE SHIBA.
In fact, they are so different that they actually very rarely make good housemates. The differences in their temperament and play style often clash leading to tension, including fighting, in the household. Shiba generally expect their space to be respected, one of the primary traits that's led to their "cat-like" reputation, while shikoku have no real, innate concept of personal space when it comes to other dogs. They are noisy and antagonistic, things that shiba don't particular appreciate and will likely correct. Shikoku however are not a breed to back down so when corrected, they will "correct" back with both breeds continuing to react until a fight ensues. As a result, most experienced owners would recommend against keeping the two breeds together unless they can guarantee that one or the other is well-socialized enough to avoid constant conflict. Ultimately it is possible that they might learn to live together peacefully and tolerate each other but only in a handful of rare cases do they actually get along and enjoy each other's company. Certainly not a breed I would look to hoping to find your shiba a "bestie." Worst case scenario, they would be unable to be together at all as they mature meaning the owner would have to crate and rotate the dogs, strictly managing their interactions. While that might not be the definitive outcome of bringing home a shikoku while already owning a shiba, there is a high probability of it occurring and something anyone interested in shikoku should keep in mind. If the idea of crating and rotating seems too unappealing, I would suggest possibly another shiba or a more compatible breed than a shikoku. Of the medium nihon ken, kai are probably the safest bet. The moral of the story here is that no two breeds are identical in temperament so please don't assume you're prepared for a new breed just because of a previous breed you've owned. It's not fair to either. Do your due diligence and RESEARCH each breed thoroughly before committing to getting one.