The abbreviation for four times a day is:

The abbreviation for four times a day is "q.i.d." This stands for "quater in die," which is Latin for four times a day. It is commonly used in medical prescriptions to indicate the frequency of medication administration.

References:

Medical Abbreviations: 32,000 Conveniences at the Expense of Communication and Safety by Neil M. Davis

Institute for Safe Medication Practices (ISMP) - Abbreviation Guidelines