Over Clocking will void your warranty on the unit, please be careful 
By the way i dint know over clocking voids warranty as it states -+10% overclocking,underclock in your website on s4. what i did is like 5% increase in frequency i guess.
+- value is the tolerance range for the hashing power in claimed calculation power. With Stock setting it may go up and down up to+- 5%
Actually the exact phrase on the BitmainTech shop product page is "Over clocking: About 10%, but not guaranteed". To me that suggests you can do it but it can't be guaranteed that it will work properly. Also, I have looked through the BitmainTech website and can not find any warranty information on their website or in their manual.