Hmm, interesting topic. I always felt that the +4 was maybe a bit of a heavy modifier for scope.
However, as a shooter, I got to try the difference at 'long' and 'very long' range!
Our club shoots at 200yds and 500 yds. Most of my shooting up until now has been at 200.
I use a Saiga M3 (think modern, civilianised AK) in .223 calibre with a x9 magnification scope. The other rifle I use is a Mauser K98 (WWII German standard issue rifle in 7.92mm or 8mm Mauser calibre).
At 200 yds, I get much better groups with the scoped Saiga than I do with the K98 on iron sights, but to be honest, I still get all shots out of a detail on the paper and regularly in 'the black' with the K98 (occasionally a bull, but that is more luck than ability!). This would imply hits on a torso sized target (assuming said torso wasn't moving!)
Last shoot, we then moved back to 500yds. With appropriate adjustment on the scope for bullet drop, I was still getting the black with the scoped Saiga, and generally reasonable groups. With the K98, I was getting about 30% of shots on target.
I am not military or ex military, simply a civilian shooter.
My conclusion from this is that the benefit a scope brings is quite large! However, the benefit is more noticeable at the longer range(in this case very long range)! I would therefore be inclined to give a bonus for using scopes as +2 at long range and +4 at very long range (which may well have the effect of offsetting the negative modifiers received by the wepaon at those ranges, but I would be inclined to allow it to be a postitive modifier overall, as it certainly aided me! I would imagine a trained marksman would reap even more reward, which is why I would still allow the DM to run positive).
Your mileage may vary, this is simply my two credits worth!