What country are you in ? From http://support.apple.com/kb/HT4539 :
Automatic Downloads for music and downloading past music purchases is currently only available in Argentina, Australia, Belgium, Bolivia, Brazil, Canada, Chile, Colombia, Costa Rica, Cyprus, Czech Republic, Dominican Republic, Ecuador, El Salvador, France, Germany, Guatemala, Honduras, Ireland, Luxembourg, Malta, Mexico, New Zealand, Nicaragua, Panama, Paraguay, Peru, Slovakia, Spain, Switzerland, the United Kingdom, United States, and Venezuela. iOS 5 or later is required for iPhone 4 (CDMA Model)
If you aren't in one of those countries then you can copy them from the iPad to your computer's iTunes via File > Transfer Purchases and then sync them to the iPhone.