As you probably know, things like 3D games and wifi are energy hogs. But if you're thrifty with energy, I'd be willing to be that the battery needs changing - especially since your Touch is four years old. Your battery will need replacing when it's unable to hold a charge sufficiently.
You have several options. You can go to your local Apple Store, which can replace the battery (for a nominal charge. Or, visit iPodJuice, which can replace the battery and for around $40 - which worked great for me.
A properly maintained iPod battery is designed to retain up to 80% of its original capacity at 400 full charge and discharge cycles. You may choose to replace your battery when it no longer holds sufficient charge to meet your needs.
If you search the iPod Touch forum for battery you will find many posts on short battery life after iOS 4 updates. The multitasking seems to use more battery when the iPod is sleeping. I just place my iPod in airplane mode when I am not using it. That really help