>>102080879Proposal:
Numbuh Zero says blurpleberry is the closest commercially available flavor he knows of to the Fourth Flavor.
Numbuh Five says it tastes "just like" another flavor.
This other flavor cannot be blurpleberry, because as a known commodity she would have been disappointed, not elated, at its similarity.
Concealment of the Fourth Flavor would be pointless if it could be so closely approximated in a known, publicly available product.
Once tasted, the Fourth Flavor can be identified as a flavor not normally used in ice cream but, once done so, is elevated to the status of a primary flavor alongside the three known ones.