pH 4 is easier to preserve than pH 5.
If you have pH 4, more of it will be in acid form and it will not dissolve in water and will precipitate.
What i want to say is, if 0.07% DHA in acid form can’t preserve your product, then add another anti fungal preservative with it. Increasing DHA won’t help.
Look at the commercial preservative blends that has DHA in it. There isn’t a blend that gives you more than 0.07% DHA in final product at maximum recommended use.