O known iH8sn0w announced the release of untethered jailbreak for iPhone 3GS and Appliances A4s running iOS 6.1.3 or 6.1.5.
With the sudden and without warning its release evasi0n7 (iGuRu.gr introduced it first and by far), the untether package was also released via Cydia for 3GS / A4 6.1.3 and 6.1.5.
To download the package you should already have the tethered jailbreak from the last one redsn0w or sn0wbreeze. Then everything is simple. Open Cydia, update your packages and you will find it by searching for “p0sixspwn".
Sn0wbreeze will be updated shortly with the untether jailbreak.