If you download firmware for "Archer C6 v3" and flash it onto a v3.20, the router might fail to boot. Conversely, v3.20 firmware will not work on v4 hardware. Always, always verify your hardware version before proceeding.
The Archer C6 v3.20 supports TP-Link HomeShield (security, QoS, and parental controls). The recent firmware patches a bug where traffic statistics would reset every time the router rebooted. tp link archer c6 v3.20 firmware