Many players avoid using the 4th finger right after or before the 3rd finger, since they are not 100% independent - if you stretch out your hand palms up, you can bend all fingers one by one except the pinky, which will drag along the third finger. If you play with a “blues vibrato”, you usually angle your fretting hand 45 degrees to the frets (less on the low strings, more on the high), which sometimes puts the pinky outside the fretboard. Piano players often prefer fingerings that avoids the succesion of pinky and ring finger.
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