In this work 4H-SiC bulk growth experiments have been performed to study the generation of three types of polytype instabilities during 4H crystallization from the vapor phase: Island growth of other polytypes during the initial stage, circular ring sector inclusions generated at the edge of the facet and complete conversion from 4H to 6H growth generated on the facet. The dependence of these instabilities on the process parameters seed surface temperature, inert gas pressure and axial temperature gradient have been investigated. Types i and ii of polytype instabilities (6H/15R) were observed to be not stable and overgrown by 4H. Furthermore type i could be avoided by a seeding procedure with low supersaturation. No dependence on pressure or axial temperature gradient was noticed for the type ii polytype generation mechanism. Concerning the complete conversion to 6H, 4H stability showed no correlation with the argon pressure but we found a critical temperature for conversion, which depends on the axial temperature gradient. Far low axial T-gradients the critical temperature decreased.