IPhone 4 Home Button Assembly
The iPhone 4 has miraculously few moving parts. A benefit of almost exclusively operating from a touchscreen. The front of the device only has a single button, but like all buttons, it is subject to failure.
Lots of failure apparently. A stuck or sticky Home Button is the third most common failure on an iPhone, behind a dead battery and a broken panel. Thankfully, there is no reason to deal with a stubborn button. You can replace your home button cheaply and easily.