Shields in D&D have never been done right. Ther's a reason nearly everyone on the ancient/medieval battlefield used them! +4 for a large shield sounds about right to me. More might even be better/more 'realistic'.
Its true, there was good reason to have a shield on the ancient battlefield. But as much as that is true if you make shield too good then everyone ends up using them and there ends up being no point in having 2-handed weapons, which becomes a "numeric pigeon-holing" effect.
I want that when players are trying to make a decision of sword'n'board vs two-handed vs dual weapons, that it is a decision and not a forgone conclusion. Historical accuracy be damned. +4 on a d20 scale is a bit too much for my tastes