Yes, there is a penalty for doing that.
You are required to cover 100% of prior year's tax (110% if your adjusted gross income is $150,000
or more ($75,000 if married filing separately)) or 90% of your current year's tax in a timely manner. The key is the timing aspect. Withholding is deemed to be paid in evenly throughout the year. Estimated tax payments are deemed paid on the date of payment (hence the reason there are quarterly estimated tax payments).
You can get an exception to the timely aspect if you have uneven income. In other words, if all of your income is earned on 8/15, for example, you can complete Form 2210AI when you file and show that all of your income was earned on 8/15, so you won't have a penalty if you paid all your estimated tax payments by 9/15, rather than earlier in the year.
As a general rule, if you try to pay everything you owe for the year on April 15th, you'll be hit with an underpayment penalty.
