So April 8 2021 was a day of shock for Singapore.
The presumptive PM-apparent/PM in waiting/PM designate Mr Heng Swee Keat announced that he would not be taking up the PMship after all, and would rather another younger person be the PM-designate. His reason, or his publicly stated reason was that he was 60 years old this year, and by the time the Covid-19 crisis was settled he would be close to his mid-60s. He would have limited time to provide leadership and stability for Singapore.
In other words, he had already been selected as the next PM of Singapore. If he had just coasted along, taking things one day at a time, and just making sure that he doesn't screw up big time, he would be Singapore's 4th PM.