The difference between the third and fourth ionization energies of scandium is much larger than the difference between the third and fourth ionization energies of titanium. why?

This is because removing three electrons means third ionization energy would leave scandium with a full valence shell that is 3s²3p⁶ and removing a fourth electron would begin to remove electrons from a full valence shell which is very energetically not favorable.

Removing a third and fourth electron from titanium is bringing Ti closer and closer to having an empty 4th energy level and a completely full 3rd energy level. Removing the 4th electron would be very energetically favorable because it would result in a full valence shell.