Recast characters are those who were originally played by one actor, but then the role was recast and played by another actor.
Three major roles were prominently recast by Season 4: Ser Gregor Clegane, Daario Naharis, and Tommen Baratheon. Gregor Clegane was recast for Season 2 and yet again for Season 4. Daario Naharis was given a full introduction as a major new character in Season 3 with significant dialogue and primary interaction with Daenerys Targaryen, but was then recast. Tommen Baratheon had appeared as a third-tier character in Season 1 and Season 2, and was recast for Season 4 by an older actor (who had coincidentally appeared on the show briefly in a different role as Martyn Lannister in Season 3) to fit his more prominent role and mature storyline.
Beric Dondarrion and Rickard Karstark were more of "placeholders" before their main introduction in later seasons, however, they did have speaking lines and were also credited, so they count as "recast".
Sort of a gray area are characters who were initially played by uncredited extras, then officially cast and introduced. For example, the first High Septon (Before the riot of King's Landing) technically appeared in Season 1, but with no speaking lines and not identified as "the High Septon" except by his costume. Similarly, Selyse Baratheon technically appeared in the Season 2 premiere, but only appeared in a crowd shot, had no speaking lines, nor was she identified; we only know from casting information that this was actually supposed to be Selyse. Given that we want to avoid confusing TV-first viewers, we'll avoid complicating matters and not include either of these two under "recast" characters. TV-first viewers wouldn't even know that these characters had appeared unless we pointed it out.
For the time being, Jaqen H'ghar of the Faceless Men will not be considered to be a "recast" character. A hooded figure did appear in the prisoner cart during the Season 1 finale, uncredited, but this was another "placeholder" before his full introduction in Season 2. Considering his face-change in the Season 2 finale, it was still voiced by the same actor and the different face was an in-universe aspect.