As Joe mentions, if the mash pH is falling into the desirable range, an acid addition should not make the beer sour. But if the amount of acid is large, the quantity of the anion added with that acid may have an effect on flavor. With Acid Malt, the lactate ion can have a flavor effect. Four percent addition might have a flavor effect.
Joe also raises a good point for those of you needing to acidify your mashes with Acid Malt. If this is the case, its probably important that the sparging water also be acidified to reduce its alkalinity to avoid tannin extraction during the sparge and raising the overall pH of the runnings in the kettle.
The phenomena of 1 percent Acid Malt addition causing a 0.1 unit pH suppression is interesting. As any of you who have played with acidifying an alkaline water know, the pH changes little during the early incremental acid additions and then drops like a rock at some point. This is the bi-modal response of initially consuming the alkalinity and then the true acidification. In the case of the mash, it provides the bulk of the alkalinity consumption. That allows the Acid Malt additions to produce the relatively linear pH reduction with respect to quantity added.