Using an iPhone in Japan

I will be in Japan for three months this summer, and I was wondering if anyone could tell me more about possibly purchasing SIM cards. Right now I have an old iPhone 4, but I'm not sure whether or not it's unlocked. I also have an iPhone 5, but I have a contract with a carrier in the United States currently. Is it possible to purchase a SIM card in Japan for either phone? What I really need is wi-fi, so I can find my way around. Any help would be appreciated!

just go rent a wifi device while you are in japan, and remember to shut off the cellular data. one time I end up with $500 bill from verizon. :(

You will need to verify that your phone is a model that can work on Japanese carriers and is unlocked (unlikely if you got it from a US carrier and haven't had it unlocked yet). Usually this is a simple process, just contact your carrier.

Data sims are available in Japan on monthly capped and monthly unlimited plans. Here's one of the cheaper websites with those offerings:

http://www.econnectjapan.com/

Another option will be to rent a pocket wifi like the previous poster suggested. This is easier than dealing with SIM cards, but quite a bit more expensive.

Alternatively, if you only sporadically need data then you might just rely on public free and paid wifi hotspots. The paid ones are quite inexpensive and have good coverage in the bigger cities.