On the day they got their marriage certificate, they had four babies in one pregnancy. What happened

Fu Yao was reborn, and the first thing she saw when she opened her eyes was back on the bed where she had been caught cheating by her scumbag boyfriend.
In her past life, she went to Beijing to get her marriage certificate with great joy, only to be tricked by her scumbag fiancé into sleeping with a drugged stranger. She was caught in the act and her reputation was ruined! She was sent back to the countryside, forced to have an abortion of her quadruplets, and spent her whole life as a maid for her scumbag fiancé's parents.
It wasn't until the day the two old people breathed their last that she realized it was all a scheme by the scumbag couple. The scumbag, in order to climb the social ladder, had already married a wealthy woman from a prominent family in the city. To keep his marriage a lie from being exposed, and also for his own future, he sent the Chinese medicine Cheng Ji to Fu Yao's bed—killing two birds with one stone.
In a fit of rage, Fu Yao picked up a butcher's knife and stabbed the adulterous couple to death.
Looking at Cheng Ji sleeping soundly in front of her, her eyes reddened. Let whoever wants to be the sucker in this life, she's not going to play along anymore.
She was determined to throw herself into the arms of the powerful professor Cheng Ji.
"Professor Cheng, we absolutely must get this certificate."
They agree to a three-year marriage and then leave each other alone.
Cheng Ji believed that getting the marriage certificate was merely a temporary measure and a form of compensation.
But Fu Yao's words made Cheng Ji unable to contain himself.
"I'm pregnant with your quadruplets."
"But wife, I... I'm childless..."