My iPhone4 is weird. The bottom touchscreen can be slid (to unlock) but it's really hard to tap the app in the bottom area. It seem the bottom screen is not sensitive but I can slide to unlock. What's wrong with my iPhone?
Is My iPhone 4 Touchscreen Malfunctioning?
iphonetouchscreen
Related Solutions
Most likely, cable nr. 2 (responsible for the touch-input) isn't connected properly. Try to re-open your iPhone, disconnect an re-connect the cable.
If that's not working, you have probably damaged the cable or something else.
It's, like Gerry said in the comment, next to impossible to determine a cause without physical access to the device.
Sparky works for testing multitouch, so you can try different areas of the screen by holding one finger in a place you know works and move the other elsewhere. If the line breaks, then the second finger is in a location that doesn't work. You can test it full screen (without the Safari UI but the status bar is still present) by adding it to the home screen.
Best Answer
It is almost certain that you have a hardware problem, something wrong with the actual touchscreen. Since the iPhone 4 is less than a year old it's still under warranty, Apple will fix it for free.
If you haven't done so already you can try a complete restore of your iPhone, erasing everything and returning it to it's brand new state (after backing it up). Odds are not good that this will fix it, but it's worth a shot, especially if having Apple repair it is a hassle.
Apple explains how to back it up and restore it here:
iTunes: Backing up, updating, and restoring your iPhone, iPad, or iPod touch software