To earn the Breaking the Limit trophy in Assassin's Creed Odyssey, you must have purchased at least the first episode of the Legacy of the First Blade DLC.
You also need to have completed the second main quest in this new questline, Shadow of a Legend, to get the Pride of the Lion sword. You need to use this sword for the challenge.
You also need to have the Rush Assassinate ability upgraded to Level 3 in order to chain four targets at once. Note that you can reset all of your ability points on the ability menu if you don't have the points available to upgrade Rush Assassinate.
Now all you need to do is find any camp, outpost or building with guards from which you can reliably get four strikes on different targets. When you Rush Assassinate by aiming at a selected target, you will instantly appear behind the selected target and stab them. While in slow-motion, you can pan the camera around and locate another target and press the Rush Assassinate ability again.
If you're struggling to find four targets within a single field of view, consider aiming for two tougher targets that wouldn't go down in one hit. To chain four times you can actually zip back and forth between two targets, as long as they weren't killed by the first strike.
Once you have chained four Rush Assassinates in a single flurry you will unlock the trophy.