So you have two buckets with volumes of 4 and 3 litres. This means you can add/subtract 4 and 3 as many times as you want, starting from 0.
Ex. filling the 4-bucket adds 4, and filling the 3-bucket using the 4-bucket is subtracting 3 to make 1. This also adds 3 to the 3-bucket.
We have reduced the problem to adding/subtracting 3, 4, or one of the sums to two sums with a starting value of 0.
Now, it is commonly known that the set of integers is closed under addition and subtraction. This means that when you perform those operations to integers, you will get only integers as the answer. It is also quite easy to see why.
Therefore, it is impossible to get anywhere closer to pi than 3.