It is time for violin and viola players to learn — First Position Open-String Pinky Equivalents.
Since Violin and Viola Strings are tuned in fifths, the pinky can match the same pitch as the open strings.
Here is the first position pinky rule:
Default 6
Pinkies performed on the string directly to left of an Open String, can create the same pitch as the Open String to its right. 
Left Pinky Note Options in First Position for Violin and Viola
Default 3
Default 4
Default 1