The script for the movie that nobody asked for (and everyone is actively asking against) will be written by Oren Uziel, who is best known for writing the script for 22 Jump Street (the sequel to an adaptation of a TV show) and the upcoming Mortal Kombat screenplay (a reboot of a franchise based on an adaptation of a video game), and neither Tommy Lee Jones nor Will Smith will be reprising their roles.
Jones' departure is expected, since they were out of ideas for how to shoehorn him back into the franchise (having already de-erased his character's memory in MIB 2 and then replaced him completely in MIB 3), but what about Will Smith? It can't be that hard to write another end-credits rap song, right? Well, despite the fact that MIB 3 raked in over $600 million, the studio hasn't been able to make much money on the franchise because Tommy Lee and Will demand such high salaries. With a reboot, they can still cash in on the series' goodwill while using cheaper actors to play the leads, so don't be surprised if Agents J and K mysteriously transform into Bow Wow and Tom Berenger.