The double bass is so big that the distances between the notes are very big too. To go up from one note to the next (in semitones) they use finger 1, then 2, then 4. Finger 3 is just helping finger 4 to press down on the string. There are a few players who use a different fingering system: using fingers 1, 2, 3, 4. This means that the hand has to stretch a lot. It is difficult to play in tune like this, although it saves having to change the hand position so much.