Well, for sake of apples to apples comparison, 4000 meters is only ~ 2.5 miles... So, that distance is *quite* near (relatively speaking) in RL.
If you're flying Cessnas into a G.A. airport, you'd typically set up your traffic pattern for a 1 - 2 mile final, but depending on traffic, and aircraft performance, it can be even larger (3+ miles), and I'd wager that anyone who has significant RL light aircraft flying experience would likely agree that inside of that distance, not only can you see the other aircraft in the pattern, you can also usually ID them as well (at least in terms of type).
In terms of just spotting other aircraft, I'd also agree with Chill's estimates as being pretty typical overall, and substantially further away than a couple miles.
So, in that regard, in flight sims, it's generally much more difficult to spot planes than it usually is in RL, and it's why most of them have zoom functionality -- a PC depiction just isn't as good as the trained human eyeball. Mind you, in both environments, spotting planes IS a skill, and it takes plenty of practice.
In many of the WWII cases, obviously, those guys had very good eyesight, but also, it helped a lot to know where to look, and of course, the altitudes were a lot higher, and the planes were metal (and larger than our kites), and would flash in the sunlight. Further, they were often in gaggles (rather than loners), so all those things would help... Still, impressive all the same, and there were definitely plenty of stories about certain guys who would consistently spot enemy a/c before anyone else in their flight would see them, even while they were still quite distant.