How to tell how much data I have left on my iPhone?

Ok I'm a first time smart phone user and I just got an iPhone (yay!) my dad told me I have a 4GB data plan (which I'll mostly use for safari) I looked on settings and went on usage and it showed like sent and received. Which is which? Is sent how much I've used and is received how much I have left? I don't wanna run my parents over if i use my iphone somewhere! ***Also! If I use my house's Wi-Fi on my iphone will it use up my data? I just want to know so i don't have to lug around my ipod touch as well.

I recently got an iPhone 4s and went through the same dilemma (except I only have 1GB of Internet Data).To answer your first question, the amount of internet data that you use is determined by adding BOTH numbers - received and sent. And you should keep in mind that 1000 mb of used data is roughly the same as 1 GB, so you can use approximately 6000 mb.
Also, if you use your house's Wi-Fi, it will not use up your data. Since you aren't receiving or sending data from your carrier, none of your data would be used.