I had syphilis in 2014. The RPR was 1:128. Got 3 penicilin shots. My most recent blood test shows RPR 1:4. does that mean I got reinfected?

Probably not. Successful cure is defined by a 3-dilution decline in RPR or VDRL titer. Your decline is 5 diultions, so probably all is well. However, if your titer previously declined to a lower level, i.e. negative or under 1:4 and now has risen to 1:4, reinfection is possible. Best to discuss with the doctor or clinic where you were treated, but most likely no problem.