While R0J0's answer is correct, I thought I'd take a moment to explain why this works.
random(x) returns a random number between 0 and x, Including random decimal points. So therefore random(4) could return 0, or 1.75 or 3.14 or 1.1217597243 or whatever, as long as it's a positive number and below (but not including) 4.
This is where the int() comes in. By using the int() expression he converts the number to one that does not have fractions, it's effectively the same as using floor(). 1.75 would become 1, 3.14 would become 3, and there are now only 4 possible numbers that int(random(4)) can produce: 0, 1, 2 or 3 (each with an equal chance).
Now by taking this and multiplying it by 90, your possible angles for the bullet are 0, 90, 180 and 270, which does exactly as you wanted it to.