I would put both him and Vegeta lower than 4,000 when they first arrive. Note that despite standing a few feet from him, nobody picks up on Vegeta's (or Nappa's) strength. Piccolo only figures out Vegeta's (kinda) when he yells at Nappa and Nappa obeys.
So, Nappa arrives at resting power level maybe as low as the one thousands. He powers up to the 4,000s, enough to wreck Tien and overpower any of Earth's fighters one-on-one, but still take a good bit of damage, especially from Piccolo and a Gohan rage burst. He wigs out over Goku's 5000 reading at this point.
Goku arrives and powers up to over 8000. Nappa freaks the hell out and blindly attacks. Goku punts him away, then Nappa powers up to the 6000s (I suspect he was still thinking the scouter reading was a mistake). Goku knocks him around again, and Nappa powers up again (8000-ish), fires a big attack ("Gigantic Storm" in the PS2 games, I think) then immediately goes to hand-to-hand with Goku, resulting in a draw. This is where Goku says 'this could take all day' (Vegeta too, actually).
Nappa should be a little weaker than Goku, because Goku was able to block his ultimate attack at close range.
Herms wrote:The only problem with that is that Vegeta says he didn't learn the trick of controlling one's battle power until he encountered Goku and the others on Earth, so presumably Nappa couldn't do it either. Though that leaves it unexplained just what Nappa is doing all those times when he generates an aura.
My Viz translation says "how to
conceal my true power!" (emphasis mine).
So, Nappa could control it, raising and lowering, but if you turned a scouter on him, it would always read, say, ~8000, unless he trained or slacked off or something like that.
What Vegeta learns is how to actually hide his power from the scouter (which probably carries with it some more experience in fine ki control, but Vegeta's not the kind of guy to bother with that).