Which of the following represents an unacceptable sig for topical medications?

The choice indicating "Instill into affected eye" is the correct representation of an unacceptable sig for topical medications. Topical medications are intended for application on the skin or mucous membranes, and they are not meant to be directly instilled into the eye unless specifically labeled as an ophthalmic formulation. Using such language can lead to unsafe practices, as it suggests a mode of administration that is not appropriate for standard topical agents.

In contrast, the other options provide ambiguous but acceptable directions for use. "Apply to skin" denotes a general method of application that is consistent with topical medications. "For daily use" suggests a frequency of application that is common in topical treatments. "Reapply every four hours" also indicates a periodic application that can be typical for certain topical medications, especially those addressing skin conditions. These statements are vague yet safe if used appropriately with the right formulations.