The rule says that since we have 4 fretting fingers, we should apply those 4 fretting fingers on 4 consecutive frets. For example, the major scale to the left is within a 4 fret range, so we use one finger for each fret. The fingering for this scale is obvious. But what if its in a 5 or even 6 fret range.

The major scale to the left is within a 5 fret range. The majority of the notes are within a 4 fret range though (the 4th, 5th, 6th and 7th frets), so use the one finger per fret rule over those 4 frets. For the extra fret (the 3rd fret), stretch your 1st fretting finger down one fret to reach it. Your left hand and other 3 fretting fingers should stay in the same position though. In this case, your 1st finger frets both the 3rd and 4th frets.

If the extra note(s) were on the 8th fret instead of the 3rd, you would use your 4th fretting finger and stretch it up one fret. Then the 4th finger would be fretting both the 7th and 8th frets.