I always get students to play 7ths with four fingers, with three main exceptions.
D7 - F# C D
E7 - G# D E
F7 - A Eb F
The first two can be tricky for people with less 'non-slim' fingers to get their third finger on the A or B without hitting the black keys on either side. F7 is really uncomfortable played F A (C) Eb, and making the often played F > F7 change (before a Bb most of the time) is simply a matter of moving the 2nd finger from C to Eb.
Of course it's possible to play all 7ths with just two or three notes in AI Fingered mode, but four notes makes for a more 'balanced' left hand - there are suggested chords and fingerings in some books and chord dictionaries that lead to 'wandering 5th' or 'waggling thumb', neither of which are desirable. Also, if you're playing your chords in the correct inversions, four note 7ths can make chord transitions a touch smoother.