2 is good, as is 3. 4 has it's moments, but pretty clear by then the franchise lost it's steam.
It is a great example of a series getting more and more complacent and too in love with its main characters, and eventually its extended family surrounding them, turning from something relatively interesting and adult for a mainstream genre film in the first to a total love-in by the fourth (at least have Joe Pesci killed off to create some
drama! No? OK then). Number 4 is particularly terrible because it also totally wastes Jet Li in his US debut by reining him in due to the focus being on the two geriatric cops lumbering around. It would be akin to hiring Jackie Chan and then making him do CGI enhanced stunts or buddy comedy rather than lots of action showing off his physicality.
Though the fourth does feature (unfortunately) my father's favourite film line, spoken by one of the Chinese bad guys, which he insists on quoting at the most excruciatingly embarrassing moments: "I said fried
rice, you plick!"