S4 is a great upgrade.

-lighter
-thinner
-bigger battery
-better screen, text are sharper than s3
-sensitive screen works with gloves
-able to detect hovering finger above screen
-more ram, able to cache more apps, less delay when reopen apps
-way better camera quality
-can work as TV remote

It's everything the s3 is and better. All the above points are hardware, no mod you can do on s3 to achieve those feature. Not to say s3 is bad, it's great. I rather take s3 than Sony xperia z or htc one due to sd card and removable battery. I just feel weird you say got review say s4 not a big upgrade. I mean come on, bigger screen in smaller lighter body with bigger battery is a big achievement already.

When does your data plan renew the quota?
Maybe you just start use s3 when quota almost over, new phone sync a lot of things.