What happens if I skip a quarterly estimated tax payment and just pay it all in April?

You can pay everything in April, but the IRS charges an underpayment penalty for the quarters you shorted, because the tax system is pay-as-you-go. The penalty is essentially interest, currently tied to the federal short-term rate plus three points, accruing from each missed deadline until you pay. It is not a flat fine, so a small shortfall costs little while a large one adds up. You avoid it entirely by meeting a safe harbor: paying 90% of the current year's tax or 100% of last year's (110% if your prior AGI topped $150,000). If you fall behind, make a catch-up payment as soon as possible to stop the interest from growing.