What are the penalties if there is not enough money in my checking acct on Friday 4/7? I messed up, it won't be there until 4/11. Is it too late to fix?

Since there is not enough money in your checking account, then most likely your debit will be rejected. If that happens you can pay your Federal taxes owed by direct debit on the IRS website.
https://www.irs.gov/payments

You can also see if you can cancel the direct debit payment, it maybe to late but see the following.

You can cancel the transaction up to 48 hours before the date you designated the funds to be debited, call the IRS at 1 (888) 353-4537.
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If you owe State taxes, you would need to check your States website to see if you can change the date, you can also pay state taxes on their website.