Transmission electron microscopy (TEM), high resolution x-ray diffraction (HRXRD), and KOH etching have been used to study the dislocation structure of 4H SiC crystals grown by physical vapor transport (PVT). Many of the observed etch pits form arrays extending along the < 11 (2) over bar0 > and <(1) over bar 100 > directions on (0001) wafers. Plan view conventional and high resolution TEM show that both types of arrays consist of pure edge dislocations threading along the c-axis with identical Burgers vectors of the a/3 < 11 (2) over bar0 > type. The former arrays are interpreted as slip bands formed by dislocation glide in the slip system < 11 (2) over bar0 >{(1) over bar 100} of hexagonal SiC during post-growth cooling. The latter arrays constitute low angle tilt boundaries with tilt axis parallel to the [0001] direction. Evidence is presented that such boundaries can form by polygonization of the threading edge dislocations introduced by plastic deformation.