Tupac Shakur's murder trial is entering its fourth day ... and his alleged killer, Duane "Keefe D" Davis, is back in court ... and we're carrying all the courtroom action live.

Keefe -- who is on trial in Las Vegas -- is accused of planning the legendary rapper's murder and allegedly producing the gun used to kill Tupac ... though prosecutors say he wasn't the trigger man that fateful night.

He has pleaded not guilty to all charges against him ... and sources close to him say his past admissions in an interview that he was involved in the slaying are simply empty brags.

Prosecutors have called numerous witnesses over the last few days ... including one former Las Vegas cop who claims Tupac knew who shot him -- but wouldn't rat the killer out to cops.

Suge Knight's keeping tabs on the trial from state prison in San Diego, CA ... he was in the car with Tupac when the fatal shots rang out ... and he told us Keefe wasn't the only one involved in Pac's "assassination," and some of the witnesses are full of crap.

If convicted, Keefe D faces life in prison.