I tried the following : $x = - y - z + w $ so we can express every vector $(x, y, z, w)$ as $(x, y, z, w) = (-y -z + w, y, z, w) = y(-1, 1, 0, 0) + z(-1, 0, 1, 0) + w(1, 0, 0, 1)$
so the basis vectors are : $(-1, 1, 0, 0)^T$, $(-1, 0, 1, 0)^T$, $(1, 0, 0, 1)^T$
Does this make sense? Is that really the basis?