Before I answer your question, I'm going to share two sources of information I have used to calculate Blood Alcohol Content, and which I think you might find very useful:
Alcohol & Your Body - Brown University
Blood Alcohol Chart - Washington State University
Ok, now on to the calculations.
The general average that your blood alcohol content goes down per hour is 0.015
That means each hour after your last drink, you can knock 0.015 off your blood alcohol content.
Now all you need to work out is how much you drank (1 beer) and how many hours you have till the test (since you didn't specify whether 3:00 and 4:00 are am or pm I wouldn't want to guess.
But all the information you need is provided in the links I shared.
There are some super straight forward charts, and they are even divided for male drinkers and female drinkers, so you should be able to work out how long you'll need to pass an etg test.
Good luck and stay sober friend!