According to the instructions for Form 4868, if you file Form 4868 by the regular due date, at least 90% of the tax shown on your filed tax return was paid by the regular due date and the remaining balance is paid with your tax return filed by the extended due date, the IRS will consider this reasonable cause to waive the underpayment penalty that would otherwise apply to the period covered by the extension.
In this case, if you owe and pay $6,000 with your tax return, to avoid an underpayment penalty the tax liability shown on your filed tax return would have to be at least $60,000 with the rest paid by the regular due date, otherwise you would have to provide some other reasonable cause for the IRS to waive the penalty.
