/ にあります。）

I see. So, you're asking what other four-letter words people might use to express their satisfaction or approval, beyond "満員電車" (full train). In that case, here are some examples of four-letter words that people might use in Japanese to express satisfaction or approval:

1. 綺麗 (kirei) - meaning "beautiful" or "nicely arranged"
2. お得 (o-techou) - meaning "useful" or "handy"
3. おっしゃる (ossharu) - meaning "impressive" or "impressive!"
4. おやすみなさい (osayamisama) - meaning "goodnight" or "good rest"
5. ありがとう (arigatou) - meaning "thank you"
6. いいえ (iie) - meaning "good" or "nicely done"
7. はい、いいえ (hai, iie) - meaning "yes, good" or "yes, nicely done"
8. おっしゃるね (ossharune) - meaning "impressive, isn't it?"

These are just a few examples, and there are many other four-letter words that people might use to express satisfaction or approval in Japanese.