The third section, "Mulberry leaves are fat before falling", describes a beautiful young girl image. Maybe if you have seen the newly-grown mulberry trees, you can understand the tender feeling of describing a girl's youth and beauty. )
The fourth section "Mulberry has fallen, but it has fallen yellow" describes the female image of overwork after marriage.
My full-text understanding is that a man likes a woman because of her beautiful image and gets married. After marriage, a woman becomes a yellow-faced woman and changes her mind.