I been to Japan twice.
3/4 Nights its a bit stretching it. Looking on google it's an 14 hr flight. Personally I had worse jet lag going going home.
The locals are really friendly. Most people are willing to help. English/Korean/Chinese signs are everywhere in public stations. That being said you can always use Google Translate.
Make sure your phone is unlocked. I know USA phones are on weird network bandwidths. Use this site to see if it's compatible. You can honestly just find a convenience store in the airport and get a data sim. If you are more ambitious you can ship a sim card to the post office in the Airport to pick up. Plans
I brought a bigger battery pack through the border. You will be fine.
If you have any other questions feel free to ask.
Bring cash, 80% of Japan is Cash only.