Part 5?

In 2015, Roger Murtaugh will be 78 years old and Martin Riggs in his early 60's. Yes, that's the approximate ages if you go by the first film. Riggs was in Vietnam as well and he would have been born no later than 1952 with what he mentioned. And Murtaugh celebrated his 50th birthday in December of 1986. Yes, 1986, since the movie was released in the spring of 1987. Unless it's set in the future.

So Riggs is one tough cookie that still keeps going and hasn't died of lung cancer or cirrhosis of the liver from his reckless life on the edge. Maybe marriage and raising a kid has made him a more stable and better person. And his son is now about to graduate high school.

So a sequel? Well, possibly. People are working into their 70's in 21st century America, even in tough fields like law enforcement. And you have old farts that talk about retirement for years, but can't because they need to put their kids through college, still have a mortgage, or don't have enough saved up.