docs: add TP-Link TL-WR940N v4 to hardware list

docs/index.rst

@@ -188,7 +188,7 @@ ar71xx-generic
   - TL-WR841N/ND (v3, v5, v7, v8, v9, v10, v11)
   - TL-WR842N/ND (v1, v2, v3)
   - TL-WR843N/ND (v1)
-  - TL-WR940N (v1, v2, v3)
+  - TL-WR940N (v1, v2, v3, v4)
   - TL-WR941ND (v2, v3, v4, v5, v6)
   - TL-WR1043N/ND (v1, v2, v3)
   - TL-WR2543N/ND (v1)