How to block calls on iPhone 4?
The call blocking feature was added in iOS 7, which your iPhone can run.

iOS7 allows you to block calls and messages from particular contact or number. This feature, included by default, even allows you to block messages and calls performed on FaceTime. 

Procedure:  


Open the Contacts app
Select the contact you want to block and scroll to the bottom of the contact info page.
Click "Block Contact".