The polymer poly(4-methyl pentene-1), P4MP1, displays an unusual pressure–temperature phase diagram. The previous exploration of this phase behavior through X-ray diffraction has been extended through high-pressure calorimetry. The resulting phase diagram displays a melt area, the common tetragonal crystal phase, and a high-pressure phase of hexagonal symmetry. Below the glass transition temperature of the amorphous fraction, the tetragonal phase can be disordered by application of pressure in what seems to be a frustrated transition that leads to a conformationally disordered glass. The transitions between the phases, previously postulated on structural evidence, are supported by calorimetry. Observed are the following: (1) isothermal crystal disordering by increasing of pressure; (2) isobaric crystal perfection on heating, and disordering on cooling; (3) sign inversion of the pressure coefficient of the melting temperature. Equilibrium and nonequilibrium features of the phase diagram are discussed.