Lighter Peanut Butter Frosting

In the bowl of a stand mixer fitted with the paddle attachment, beat together powdered peanut butter, milk, and 1/4 cup confectioners' sugar until well combined. Add butter; beat on high until smooth, about 2 minutes. Add vanilla, salt, and remaining 3 1/2 cups confectioners' sugar; starting on low speed and gradually increasing to medium-high, beat until fluffy. Use immediately, or store in an airtight container in refrigerator up to 1 week. Return to room temperature and beat until smooth before using.