Four radial V.B. are found in : -

In roots, xylem and phloem lie on different radii alternating with each other; such vascular bundles are called radial.
Dicot roots usually have 2 to 4 xylem bundles (diarch to tetrarch); four radial bundles (tetrarch) is typical, e.g. gram, pea.
Monocot roots are polyarch, with many (often more than six) xylem bundles.
Stems have conjoint bundles (xylem and phloem on the same radius), not radial bundles.
So four radial vascular bundles are found in the dicot root.