can someone be charged witha murder that someone else took the charge for 4 years ago ?

Yes.  Proving it is a different matter entirely.  The racketeering charge is most likely the reason the charge would survive.  Racketeering involves an ongoing conspiracy to commit crimes.  If they believe you husband conspired with the person who admitted to the murder he could be guilty under a conspiracy liability theory.  Additionally, if there was an ongoing course of criminal activicty he could be guilty under accomplice liability.  It sounds like a serious case and before talking to anyone about it he needs to consult with a defense attorney.  Racketeering may not be as serious as murder but both are serious crimes and need attention.