How do I know if there will be a penalty if I don't do four equal amounts for tax payment (1040-es) ? I can't estimate in advance four equal amounts.

You do not have to make equal payments. The program automatically generates them as equal, but you are to pay your estimated taxes as you earn your income. I you have zero income in Q2, but $100K in Q4, you will pay zero in Q2 and make a large payment in Q4.

Form 1040 ES Instructions says "You can pay all of your estimated tax by April 15, 2019, or
in four equal amounts by the dates shown below." -> Is this a SHOULD or MUST ?

It's a should. It's perfectly acceptable to use the "annualized method" and pay your estimateds when the money is earned.

Are you referring to " annualized income installment method" ?

Yes.