Beyond the first four chords, these two diatonic chords can be used to expand your progressions:

ii chord - This chord functions like a predominant, and has a strong pull to the dominant as well as to the tonic chords. It has a  melancholic, reflective feel similar to the vi chord, but with less stability within the key.

iii chord - This chord has a weak relationship to the tonic. Because it shares notes with the tonic and dominant chords, it doesn’t lead strongly to either; instead, it feels somewhat restless. It experiences the greatest pull to predominant chords, and can smoothly transition into the dominant chord.

The only other diatonic chord left - the vii° - is the dominant chord’s weaker counterpart, and it doesn’t serve as strong of a role within diatonic progressions. Because its diminished quality carries less stability than major and minor chords, it is not commonly found within diatonic chord progressions.