Step 1: Jailbreak your iPhone 4S with absinthe. Check out this video tutorial if necessary.
Step 2: Open Cydia and install Zephyr through the BigBoss repo.
Step 3: Restart your springboard, then start enjoying your multitasking gestures. Adjustments to Zephyr is available in the extensions in your Settings app.
Be sure to check out the other best Cydia tweaks for your iPad.