I think Ulnar IV IS the king in FftD. The gazetteer section of the core rules for LW suffers in this regard because the game was initially to be set in 5050, and when they made the decision to set the campaign in MS 5000, they forgot to change the names of the rulers.
The appendices at the end of Legends call him Ulnar V, so it might be that the gazetteer is not as out as it seems; certainly Vassagonia's stated ruler isn't the one ruling in MS 5050.