Well.... sure enough... the factory OE shackle length is 4" center-to-center... just measured my old original spare one... and compared it to another that's never been removed from a Challenger... 4".
The aftermarket (Moog and/or Dorman) are shorter at 3.75" (1/4 shorter!)..... Moog is pretty dominant in the aftermarket... not certain about the Dorman. TRW was likely the OE supplier, as they (TRW) had dominance with the big-3 for chassis/suspension/steering components on the assembly lines and OE service parts.
So.. IF you'd want OE factory original rear shackles, you have to have the ones that are at 4" center-to-center.
IF you want to slightly lower the rear of the car and help with improved handling (many other factors also required), OR else, IF 100% factory originality isn't within the picture, then install the aftermarket shackles... at 3.75" center-to-center length.
If it really matters to anyone, hopefully a phone call to a supplier (or parts store) would get the answer to what you're buying.... the physical measurement needs to be taken.