What are the possible assignments in Japan as Signal?

As a 25C, if you're Airborne, most of your assignments are 112th signal and SF group. I don't think there are any 25C at 78th signal in Buckner or Zama.
However, you're an E4 25C and fall under precision retention. You probably won't be offered a reenlistment option, even if you are allowed to reenlist. In order to reenlist, HRC will need to decide whether you keep your MOS, reclass or ETS.
I suggest you take this chance to reclass.

Okinawa, Japan. You could end up in the 10th RSG, 1-1 ADA or by some God given gift attached to SF. I was in Okinawa as my first duty stationed. Loved every second of it. I was stationed with 1-1 ADA. The Op tempo was crazy high but you always had predictability. You never really questioned what happened next. There was plenty to do and always days off. Morale was also high. Greatest thing about falling under 1-1 was you only fell under a BN. If it was Japan wide it was under 10th RSG but that was rare. Everything came straight from the CG so it didn't have time to get messed up. And for the island itself. I can't say enough. Beautiful people, nicest people i've ever met in my life. The weather was hot some days but it was bearable once you got used to it. The food was amazing. It's a place you'd have to experience for yourself to really get the great feel of experiencing Okinawa and not Mainland Japan. Hopefully this help!

I'm not signal but I'm currently stationed in Japan. The signal unit here is the 78th SIG BN. We're located about 45 minutes from central Tokyo. Japan is amazing assignment and a great learning experience as a soldier and an individual.