Why does my battery last about 3 to 4 hours after a full charge?

Here are some tips for that

Increase battery life

• Turn down brightness
• wait till battery is below 20% and charge to 100% (try to do this at least once every month)
• Go to usage settings and find the main culprit Settings > General > Usage
• Double click the home button and slide the panels up on any apps you don't want running in the background
• stop location tracking apps Settings > Privacy > Location Services (you can disable apps or the location service completely)
• Reduce moving (dynamic) background Settings > General > Accessibility
• disable some notification widgets (pull down from top screen) press edit and get rid of some you don't use
• avoid bad signals and turn off wifi when in a no wifi zone (slide up and press the wifi button)
• Turn off auto update (and update when charging or when you have battery) Settings > iTunes and App Store > Automatic Downloads > Update and turn it off
• Turn off Bluetooth and airdrop unless your using it

Maintain battery life

• Avoid over charging
• don't use it while its charging

Hope this help

Likely due to normal degradation of the battery with time and use. With time and use the run time decreases.