4H-SiC implanted with high dose of phosphorus has been shown to exhibit lower sheet resistance than 4H-SiC implanted with high dose of nitrogen. In this paper, we have implanted various doses (1x1014cm-2, 2x1014cm-2, 1x1015cm-2 and 4x1015cm-2) of phosphorus into 4H-SiC in order to extract the ionization energy of phosphorus in 4H-SiC as a function of the doping concentration. Variable temperature Hall effect measurements were performed in the temperature range from 60-600K. Least square fits using the charge neutrality equation with two donor levels were used to extract the ionization energies and donor concentrations from the measured data. The ionization energies for both, the hexagonal (53meV, 49meV and 26meV) and the cubic (109meV, 101meV and 74meV) site decreased as the donor concentration (5x1018cm-3, 9.8x1018cm-3 and 3.4x1019cm-3) increased.