Polymers that generate large shape changes under electric stimulation are of great interest for many applications. Recently, it was shown that converting a small amount of chlorofluoroethylene (CFE) in relaxor ferroelectric poly(vinylidene fluoride–trifluoroethylene–CFE) (PVDF-TrFE-CFE) terpolymer into fluorinated alkyne (FA) creates P(VDF-TrFE-CFE-FA) tetrapolymers with giant electromechanical (EM) response at ultralow electric fields (<50 MV/m). We investigate the microscopic origin of this effect and show that converting the bulky CFE into small-size FA defects dramatically weakens the relaxor behavior. Importantly, P(VDF-TrFE-CFE-FA) tetrapolymers with near 2 mol % FA exhibit a diffused critical endpoint transition region at which the energy barriers for switching from nonpolar to polar molecular conformations become small. Consequently, a small change of the electric field induces a large electroactuation, which can enable novel applications. This work opens up a totally new approach to designing ferroelectric polymers that generate large responses at ultralow electric fields.