It is known that the Burau representation of the 4-strand braid group is faithful if and only if certain matrices f and k generate a (non-abelian) free group. Regarding f and k as isometries of a Euclidean building, we show that f(3) and k(3) generate a free group. We give two proofs, one utilizing the metric geometry of the building, and the other using simplicial retractions.