God you can be a wanker nismoid, you know what I mean.
Start it at 2000rpm like a normal dyno run, eg on appropriate load settings, not holding it on massive load so that it doesn't increase in rpm pre spooling the turbo. If you start it at 4k you could have been holding it there for 5 minutes before hand, we don't know. At least if you were to do that at 2k it wouldn't cheat the dyno result too much assuming of course the correct ramp rate for the rest of the run.