Minors are easy on Yamaha - just press the root key and the adjacent, left, black key and you have a minor.

Major 7th, however, will require all four fingers - no shortcut that I know of for Maj7 chords.

This is from the Lessons section of the PSR-Tutorial sit. Note: Yamaha provides players with 7 different fingering modes to select from.

SINGLE FINGER

The simplest method is to use the single-finger approach. This is the SINGLE FINGER type. Using the single-finger method, you can easily play all major, minor, seventh, and minor seventh chords:

In this mode, you need only press a single key, the "root" value of the chord, as described above to trigger any major chord.
For a minor chord, you press two keys at once, the root key and a black key to the left of that root. For example, to signal the Em chord, you would press simultaneously the E1 key and the Eb1 key (the first black key to the left of the E key).
For the seventh chord, simultaneously press the root key and a white key to its left. (E7 would be signaled by pressing simultaneously E1 and D1.)
For a minor seventh chord, simultaneously press the root key and both a white and black key to its left. (Em7 would require pressing E1 and Eb1 and D1 all at the same time.)

However, the Single-Finger method, does not enable you to play a C major seventh (CM7) or C augmented (C+) or C major sixth (C6) or C diminished (Cdim) or C ninth (C9) chords. The single-finger method would work for very simple songs, but could not be used to play more modern tunes using fancier chords.

Gary