I've asked Crytoware about overclocking Antminer up to 4GHash.

Here is the ansver:

"Hi there

Please be aware that any overclocking is to be done at the users own risk, and will void the manufacturers warranty. Overclocking by that amount could greatly reduce the expected lifetime of the device. Instructions are for educational/experimental purposes only - they are included for the benefit of those customers who enjoy modifying their equipment, but customers do so in the full knowledge that Cryptoware cannot accept liability for the results!

Its up to you to decide if you want to play around with trying to overclock these devices, we have read reports of some pretty good results being acheived. We've also read reports of broken hardware due to overclocking.

Hope this answers your question,

Regards,

The Crytoware Team"