Hello everyone,
With the unexpected release of the evasi0n7 jailbreak, we are finally able to release the 3GS/A4 6.1.3 + 6.1.5 untether Cydia package.
To grab it...
1) Have a tethered jailbreak (either via redsn0w [Point to 6.0 IPSW] or sn0wbreeze).
2) Open Cydia.
3) Click the "Changes" tab in the footer.
4) Press the "Refresh" button in the top left corner.
5) After "Reloading Data", perform a search for "p0sixspwn" (the O is a zero).
6) Tap Install.
That's it! You're untethered.