Softbank prepaid sim card

I am going to Japan for 5 months and a friend is loaning me their softbank iphone 4 while I'm over there. I'm planning to rent a prepaid sim card.
I would like to be able to use line, facebook, and gps when needed. I'm confused if I will still get charged for internet use even if I remain under the data amount?
The plan cost $1.10 a day with 4g, but it caps out to $15.50 a day when you go over.
Here's a link of the plan:
http://www.softbank-rental.jp/e/rental-plan-ib03.php

I don't know a damn thing about smart phones....

Yes, you are charged 0.32 yen per 128 bytes on top of the 110 yen basic charge, up to a maximum of 1550 yen.

I am going to Japan for 5 months and a friend is loaning me their softbank iphone 4 while I'm over there. I'm planning to rent a prepaid sim card.
I would like to be able to use line, facebook, and gps when needed. I'm confused if I will still get charged for internet use even if I remain under the data amount?

Yes, but the rates are such that you hit the max pretty quickly. If you plan to use data regularly then you'd better look at other plans or you're going to end up paying a lot for your phone service. In your case, you're probably best off with a data plan like OCN or So-net or OCN, and then using an internet phone service like skype or line to do your calling.

http://service.ocn.ne.jp/mobile/one/
http://www.so-net.ne.jp/prepaid/en/index.html