In studying a music score with fingering indicated, what does it mean when there is a circle below the 4. I know the 4 means 4th finger but am not sure what the circle means.

It might be a harmonic... that is, touch the string (with the 4th finger) but don't press it to the fingerboard.

I thought harmonics are indicated by a diamond sign.

I did a little research and found that the circle stands for natural harmonics as Cicilia indicated and the diamond sign is for artificial harmonics which is more difficult to do and are for advance violinists. Artificial harmonics uses two notes: one pressed normally and the other very lightly.