Yes, sure.
The iPhone has a normal Safari browser where you can open the website:
http://www.myibidder.com/ Or you can get the Myibidder iOS app if you feel you use it often and need more flexibility:
http://www.myibidder.com/link/ios
Yes, your iPhone 4 meets the requirements.
The App Store indicates that the app is compatible with any iPhone/iPad with iOS 4.3 or higher.
If you can download it, that means it's compatible.