The Four Riders (四駿 Shishun lit. Four Horses in the Japanese version) are the current four strongest soldiers in Daein's army.
The Legendary Four Riders (or Great Riders) were the original Riders, and they consisted of Greil (then known as Gawain), Tauroneo, Bryce and Lanvega. Three of the four left their positions. Tauroneo and Lanvega resigned after Ashnard became King of Daein, and Gawain left after his wife Elena was entrusted with Lehran's Medallion from the Heron, Lillia.
They were replaced by three other Daein generals, named the Black Knight, Bertram and Petrine (Bryce kept his position). Petrine and Bryce died in The Mad King's War, and the other two (known by other identities to other people) supposedly died in the war as well. Bertram's life was actually spared by Bastian, and Bertram eventually was restored to his normal state. The Black Knight survived the events of The Mad King's War (even if Ike defeated him in battle), but Ike finally bested him in the events of Radiant Dawn.