To unlock the Black Widow trophy in The Sims 4, you will need your Sim to outlive five spouses.
The easiest way to unlock this trophy is to create a new household with five elders of the same sex and one younger Sim of the opposite sex (5 elder males, 1 younger female for example). Send four of the elders off to work so that they are not in the way, and then keep romancing the one elder and younger Sim until they fall in love, keep going until you can propose and then elope to get married instantly.
Get the elder to work out until they are exhausted, and then you can keep woo-hooing until the elder dies of exhaustion. Once they have died, quit jobs with one of the other elders and rinse and repeat until you have managed to go through all five elders.