0.004 over 6 inches is basically nothing when you do the math, that's a taper of less than half a thou per inch of travel and most shaft work has way more error stacked up from tool push and heat than that jaw ever caused. My old shop in Gary ran two shifts on beat up Bridgeports that never saw an indicator once and we held plus or minus .002 all day on 4140 because the guys knew how their machines cut and dialed the offset in. Chasing every vise with a test indicator is a rabbit hole, next thing you know you're indicating your chuck jaws and your tool holders and you never make a chip. The real fix for taper is a good steady rest, sharp inserts, and taking a spring pass, not remounting the vise every job. You probably had a loose jaw screw or chips under the vise the whole time and the indicator just made you feel like you found something. Spend those six minutes making a test cut and measuring it instead, that tells you what the part is actually doing.