I don't think most Japanese speakers would characterize their language as having “four writing systems”. They would say three and not include Romaji in the coun

I don't think most Japanese speakers would characterize their language as having “four writing systems”. They would say three and not include Romaji in the count. This is not to say they don't use Romaji, but it's not considered part of the official orthography. There are no words or sentences that can't officially be written without Romaji. The same is true with English and emoji. We do use emoji to augment the language, but that doesn't tempt us to consider emoji part of our alphabet. The analogy is weak of course, because English doesn't have an equivalent official transcription into another writing system. But you get the point. If the roles were reversed and global culture and the Internet were all in Japanese first and foremost, and most languages based their writing on kana and/or kanji so much so that you can't avoid it when traveling, then English would get an official transcription into that.