Q: why is my iPhone 4 so sluggish?

Try This...

Close All Open Apps...  Perform a Reset... Try again...

Reset  ( No Data will be Lost )

Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.

http://support.apple.com/kb/ht1430

Neecie L wrote:

 

...Is there a way to check which apps are running and disable them without deleting them.  ...

TASK BAR  iOS: Understanding multitasking

http://support.apple.com/kb/HT4211