As per the notification of 209/03/11-CX-6 dt 15/2/2011, being exempted as per the details given below :
". Exemption from filling of ER4,5,6, returns as amended -as it is availabe to interalia assessee who paid duty of excise less than Rs.1 core including the amount of paid by utilization of CENVAT credit. or Rs. 10lacs exceeds during the preceding financial year".
Therefore you need not file ER-4,5&6 returns if your duty amount is less than rs. 1 crore per year (i.e PLA + Cenvat amount)
ER-1 return are necessary for all companies irrespective or the duty payable amount.