Updated in Cydia with the release of iOS 7 jailbreak, iH8snOw and Winocm released an untethered JB package in Cydia. Those that are using tethered jailbreak on iPhone 3GS and A4 device running iOS 6.1.3, iOS 6.1.4, or iOS 6.1.5 as of right now can install that package and be able to be untethered on iOS 6.1.3-6.1.5. Current untethered package will only jailbreak devices that can already be jailbroken tethered.
So the iPhone 3GS, iPhone 4, and iPod touch 4g can download this untethered package.
Open Cydia and hit refresh. Should see a tweak called "pOsixpwn" install this package and you should be untethered from now on.
Official release of Untethered jailbreak on these iOS should be released later on. Do not get this mixed up with iOS 7 jailbreak. This is for devices that are on tethered jailbreak like myself. I will wait till Saurik updates WB and most tweaks get updated to iOS 7 before I decide to do that jailbreak. I'm comfortable with 6.1.3 and all my tweaks working for now.
Link to his blog:
iH8sn0w.com: 6.1.3 & 6.1.5 3GS/A4 untether Cydia package