FAQ for
Lethal Weapon 4 (1998) More at IMDbPro »

No. Even though they faced the toughest enemy they ever encountered and became badly wounded directly after, at the end of the movie, they continued their work on the police force.

As of now, the answer is no. Ignoring all of the fan conversations about a fifth sequel, director Richard Donner feels like Lethal Weapon 4 had a good way to end the saga. Both Sergeants were badly wounded and discovered that they needed to slow down or they could possibly face early retirement.

However, writer Shane Black reportedly submitted a draft to producer Joel Silver in 2008. Rumour has it the plot was that Riggs was about to retire and trying to persuade the already retired Maurtaugh out of retirement for one last case so they could go out together in a "Blaze of Glory". Also it was suggested that Chris Rock's character would carry the torch for any further sequels. Apparently Richard Donner wasn't interested which may or may not have deterred Mel Gibson as a result.

It was also suggested by some that Gibson would make the film as way of winning back his public after his controversial arrest over DUI and his anti-sematic statements.

NOTE: A Lethal Weapon 5 is listed on IMDB for 2012

The most obvious reason could be that it was to show that Riggs was getting older.

Also this film was known to have been fast-tracked from pre-production to final release: filming began in January 1998 for a summer 1998 release and so it could be that Mel Gibson just didn't have time to grow his hair back.

Also, in Lethal Weapon 3 everyone was ragging on him to get a haircut as it was the 90's. So it's likely he just finally caved and decided to get with the times.