You have a 5 gallon jug and a 3 gallon jug.
You have no other measuring devices.

Measure out exactly 4 gallons of water (Assume you have a river handy, or some other limitless water supply).

Let us denote the 3 gallon jug by A and the 5 gallon jug by B.

**Step 1**: Fill B with water.

*Step 2*: Empty B into A. Precisely 2 gallons now remain in B.

*Step 3*: Empty jug A.

*Step 4*: Pour the 2 gallons from B into A.

*Step 5*: Fill out B. Transfer 1 gallons of B into A.

B will now contan precisely (5-1) = 4 gallons.

Thus, we have measured out exactly 4 gallons of water as desired.