I submitted my federal return on 4/4 with direction to pay on that date. It's 4/18 and the IRS has not yet debited my account. Do I need to contact the IRS?

No, there seems to be some delay with the IRS withdrawing the funds. Please be aware that this year you have until May 17th to pay your balance due. Also, there will be no penalties or interest if the IRS failed to make the debit on time as long as the request was done correctly on a timely filed return.

Usually, the IRS will withdraw the amount on the day you selected. If that date happens to fall on a weekend or federal holiday, the funds are typically withdrawn the next business day. Please see When will my tax payment go through? for additional information.

Also, you can view your balance and check your payment activity at IRS Paying Your Taxes.