Motla68,
The irs has sent a memo for all exempt W4's to withhold at 30%. You may find yourself in the irs crosshairs when they review the data (W3) authorized by the W4 and see that you are indeed accumulating income but haven't had deductions withheld.
I bring this up because the W4 is giving the employer permission to treat your earnings as "wages" and as "wages" income tax liabilities incur at 26USC 3101 and 26USC 3402. Exempt or not this "exempt" status is not by any means telling the irs you cant be touched.