I don't have enough personal experience with hand to hand combat with weapons to know whether the 10% (+2) bonus for attacking from higher ground does not accurately reflect that benefit. If you do (or have read credible information that it does), then I would agree with your solution.
Leaving aside the idea that personal experience might be a necessity for emulating something in a game, since clearly we'd have far fewer games if that were the case, I think we can safely both agree that "higher ground" does give some sort of advantage (small "a"). Once we pass that hurdle, then using one suggested bonus as a standard, whether established or not, seems an odd rubric. So, we're really just discussing a slight difference of opinion on how deadly we might want one portion of our game or another.
But think of it this way, if someone is far enough above someone else during combat, they are swinging weapons downward, the opposite being the case for the person below. The person above is swinging a higher percentage of the time at someone's head (leaving aside some types of monster opponents). Again, the opposite true for the person below. In your opinion, should there be a +2 for high ground *and* a -2 for low ground? Would it be more exciting for game play, for the players, to slide the full +4 (4-5 , as you mention) over to one side?
I'm comfortable with that being the Basic mechanic. That is to say, it's a decent simple math way of dealing with what, we both already agree, is an advantage . . . in the rules of the Basic game before we get into more advanced combat modules/options. I would not be in favor of the Basic rules having more granulated rules when it is already planned to design the advanced version with such in place. I like that a GM, in most cases, can simple assign Advantage/Disadvantage and be done with it. I like that it puts that mechanic in the hands of the players, quite often, for the sake of resolution.
Now, something I am looking at in the rules, as our group plays through them, is whether I would even want a more granulated set of combat rules. I've been wargaming 40+ years, a bit longer than D&D has been around, and all along side my RPGing history. I think it is fun to have combat mechanics in RPGs but if I want rules that focus on combat, generally speaking, I don't play an RPG, I play a wargame, a minis game, or the like. So, please understand, when a conversation comes up about one small aspect or another of the combat rules in an RPG, my first reaction is wondering if it can be removed entirely rather than to tweak it for the sake of "realism." The Advantage/Disadvantage mechanic goes a long way, IMO, toward slimming the rules for combat in an RPG and that's a big plus, IMO.
