Personally considering you're here for only 4 months my advice would be as long as you have a debit card which has a cirrus symbol on it DONT BOTHER with setting up a bank account. ANY AND I MEAN ANY post office will let you withdraw cash from back home and the fees are usually not too bad. It also works at 7/11. With the 7/11 and Post Office and especially cos you're in Tokyo you should not be short of places to get cash from your account in the states. Opening a bank account is a lot of hassle and time and wasted effort for such a short period. The only time that a bank account comes in to it's own is when you need to pay utility bills on a monthly basis but you won't be doing that so it's a non issue.

Secondly DO NOT UNDER ANY CIRCUMSTANCES BRING OVER A BANKERS DRAFT OR CHEQUE. My friend brought one and it took him a good few weeks to get it cashed and get his hands on the cash. The Japanese do not have very good links with the International bank system in respect of things like that and it's only in the past 2 - 3 years that nation wide cash withdrawal at post offices has become common place.

-mrploddy