I am from Japan using Softbank iPhone 4 that is gevey sim unlock. I am planning to visit the usa for a month and would like internet access mainly when I am on the road. I don't really care much for the voice part, just Internet access.

T-mobile has a 'Bring your own phone to t-moible' I think may fit what I need for $70 a month. Since there is no annual contract, can I just pay for one month of usage and not be penalize with any fees when I cancel? Also, on their site, it specifically say:

Only T-Mobile and AT&Tís iPhones, and Verizonís & Sprintís iPhone 5 will work on our network; other iPhones may have limited functionality, including coverage limitations. Capable device required for 4G speeds; capable iPhone may achieve 4G speeds in limited areas.
Will I have issue as far as getting internet access on my softbank iphone 4?