Voice to text should work on the iPhone 4. We've tested it on a number of devices and haven't seen any issues. Can you describe why you can't use it? What happens when you click on the microphone icon when asking or answering a question?
Be sure to check your settings, you may have accidentally turned the feature off. Go to: My Profile --> Settings --> Be sure "Voice to Text" is set to "on"
I have heard this from several people who have the iPhone 4 and they said the phone is not compatible with the app to have that feature. My friends who have the app on their 3GS can use the voice to text on Ask.com just fine. I am not 100% sure why this is the case.