I'll assume that your previous year's tax liability was $100k or more.
Scenario 1: If you don't file Schedule AI of Form 2210, you will have underpayment penalties for the first 3 quarters because 90% of $100k / 4 means a required quarterly payment of $22,500. If you file Schedule AI showing that your quarterly payments were sufficient to cover the tax liability was only $20k for each the first three quarters because the income was lower in those quarters than in Q4, you won't have underpayment penalties.
Scenario 2: No penalty because your required quarterly payments are 90% of $80k / 4 = $18k.
Scenario 3: In the absence of filing Schedule AI your Q1 payment is sufficient ($20k is more than the $18k required) but you'll have a $6k underpayment for Q2 and a $4k underpayment for Q3.
