Measuring with jugs

COMPUTER: I am a bomb, you have just armed me. SIMON GRUBER (over a cellular phone): I got you see the message. It has a proximity circuit, so please don’t try to run. JOHN MCCLANE: Yeah I got it. We’re not gonna run. How do we turn this thing off? SIMON: On the fountain, there should be two jugs. You see them, a five gallon jug and a three gallon jug? Fill one of the jugs with exactly four gallons of water and place it on the scale and the timer will stop. You must be precise. One ounce more or less will result in detonation. If you are still alive in five mins. . . JOHN: Wait. . . wait a second. I don’t get it, you get it? ZEUS CARVER: No. JOHN: Get the jugs. Obviously we can’t fill the three gallon jug with four gallons of water, right? ZEUS: Right. JOHN: All right. I know. . . Here we go. We fill the three gallon jug exactly to the top. Right? ZEUS: Uh huh. JOHN: Okay. Now. We pour that three gallons in the five gallon jug, giving us exactly three gallons in the five gallon jug, right? ZEUS: Right. JOHN: Now, take the three gallon jug, fill that a third of the way up. . . ZEUS: NO! He said precise. Exactly four gallons.