Welcome to iPhoneForums, ladida0311!
The iPhone 4s was discontinued by Apple in September last year, so even if you purchased it as new, your device's age is at least one year.
In spite of all this, the battery should last way longer than yours does, no matter which software is installed on it.
I assume you've turned off background app refresh, your screen's brightness ranges in the lower half, and your volume isn't as loud as possible.
Here are a few things you could try:
Reset your iPhone. This won't remove data or change settings, it might correct a few glitches you're experiencing. To reset, press Home and Power button at the same time until the Apple logo appears. As soon as it's visible, let go of both buttons. Your device will power up on it's own.
Restore your device. There could be a problem with the software. To restore, use iTunes on your computer. Make sure there's a most recent backup (iCloud or iTunes, both is possible). Then connect your iPhone to iTunes and restore it. This will also update it to the latest software (8.4.1). After restoring, reset your iPhone, using the method described above.
If your iPhone's battery life is still as bad as before, take it to an Apple Retail Store. They'll take a look at it and tell you what's causing your problem.
Hope that helps.
