iPhone 4 Power & Lock Button
Quick note about the replacement battery, if you bought from iFixit and the battery sections have a thin film over them, peel them off. They're to protect the battery during transit, and if you leave them on they might cause heating issues.
Here is a link to my question about it: Replacement battery has thin film on each section do I need to remove? plus, I just got an email from iFixit customer support that it needs to be removed.
The hardest part is getting the back plate off, I throttled my plastic spudger.