Make sure of your current iOS first. Most iPhone 4s are in 5.1.1 which is the latest.
It's soooo easy! The benefits are customizing your phone, themes, tweaks and more apps and so forth. It does void the warranty, but all you'd have to do is restore before taking it in.
im not much help as to "how to" jailbreak your ipone but i had my cousin do it. it took him less than 5 min but took him close to 30 min to show me how to do what i wanted. I basically used to have a Droid and i loved it...but there wasn't many cute cases for a Droid X!!! so i got an iphone and fell in love and having it jailbroken is even better!! you can still download apps from the AppStore and now i have cute folders/icons/txt messages/lockscreen/etc....really everything can be customized to what you like. and from i hear its super easy to just Restore if you need to have it looked at (since it does void your warranty with Apple)....no harm in getting it jailbroken and if you dont like it you can go back..........the only thing i was warned about was to NOT download a bunch of things.....i mainly just downloaded things to change the way my phone looks and havnen't looked into downloading apps through Cydia??? but i'm sure others can tell you more about it if thats what you're looking into.