I believe it's 13.3% + whatever your tax bracket is. So if it's 25%, you should owe $1,532 in federal, plus any state taxes
If he claims zero expenses with respect to the consulting income then he will have to pay $491 of self-employment tax, and $930 of income tax with respect to the $4,000 of income. The self-employment tax is 13.3% of $4,000 multiplied by 92.35%. And the income tax is 25% of $4,000 less $282 (57.51% of the $491 of self-employment tax). So his federal tax burden with respect to the consulting income is $1,421 if he is in the 25% tax bracket.