SiC single crystals were grown with the intention to produce 4H wafers using a physical vapor transport (PVT) technique. Polytype switching between 4H, 6H and 15R polytypes were observed and categorized into four different types. In particular, the mixed polytypes in the seed/crystal interface regions of the crystal boules were examined in detail. The corresponding IF changes of micropipe densities in crystal wafers associated with the polytype switching at the seed/crystal interfaces were studied using optical microscopy and KOH etching techniques. PVT growth conditions affecting polytype switching at the growth interface as well as throughout the boules in all four types of boule growths are discussed.