Is the Duty Step % (0.625) X 50% = 31.25% the problem?(I am measuring 3.8uS On / 4.2uS Off)
How can I set it for 4mS On / 4mS exactly?
your understanding is flawed and math is incorrect

Duty is Steps x 0.625% // steps = CCPR1L x4 + CCP1CON<5:4>

so:
80x0.625% = 50% <=> steps = 20x4 + 0
56x0.625% = 35% <=> steps = 14x4 + 0