1. Initial State: We start with a four-liter jug and a six-liter pot, both empty. The goal is to measure exactly one liter of water using these containers and a big barrel of water.

2. Induction Hypothesis: We will use mathematical induction to show that it is impossible to measure exactly one liter of water. Initially, both containers contain a multiple of 2 liters (0 liters each).

3. Base Case: Before the first pour, both the four-liter jug and the six-liter pot contain 0 liters of water, which is a multiple of 2.

4. Inductive Step: Assume that at step $n$, both the four-liter jug and the six-liter pot contain an even number of liters of water (i.e., a multiple of 2). We need to show that after any operation (filling, pouring, or transferring water), the amount of water in each container remains a multiple of 2.

- Filling: If we fill the four-liter jug, it will contain 4 liters (an even number). If we fill the six-liter pot, it will contain 6 liters (an even number).
- Pouring Out: If we pour out the water from either container, it will contain 0 liters (an even number).
- Transferring Water: When transferring water from one container to another, the total amount of water in both containers combined remains the same. Since both containers initially contain an even number of liters, transferring water between them will not change the fact that each container will still contain an even number of liters.

5. Conclusion: By induction, at every step $n$, both the four-liter jug and the six-liter pot will contain an even number of liters of water. Therefore, it is impossible to measure exactly one liter of water using these containers.

$\blacksquare$