What the 4 A and 8.5 mOhm ratings mean for your power rail
The SLG59M301V: That Rds(on) translates to roughly 136 mW of self-heating at full load — well within the thermal budget of the 8-FC-TDFN package if the copper pad is properly soldered to a thermal via array on the PCB.
Package and board-fit note for the layout engineer
The thermal pad should be connected to a ground plane with at least four via-in-pad to keep the junction temperature below the 85 °C ambient limit across the full 4 A load.