Is it simply because Oda had not planned for the Yonkou when OP has begun Serialization?
I don't think this is the case. Knowing how Oda generally works, with all his foreshadowing, I am pretty sure the Yonkou were not an afterthought. (69 chapters between Luffy's 'I will be Pirate King' and Arlong Park is not really a long period of time for OP.)
@Taladris put it well in the comments. Much of it has to do with the fact that the Four Emperors simply are more like literal emperors, controlling large empires and so on. Also important to note the sort of quasi-bureaucratic connotation that the term holds. Not saying that negotiations occur between them, but the Yonkou form one very vital pillar of the Three Great Powers, which has been crucial to the balance of power in the current world (and is something that the heads of the World Government worry much over).
On the other hand, Roger was an absolute wild 'un who surpassed all of his contemporaries through his astounding feats, such as discovering the final island on the Grand Line. There simply wasn't anyone with equal footing (keep in mind, we are not talking about power levels and strength/calibre here, but rather in terms of accomplishments, which was mostly what permeated to the far corners of the world; there was no one comparable). Moreover, his epithet 'Pirate King' wasn't conferred to him by the World Government or some other authority figure. It was a unanimous recognition of his greatness by the world as a whole and especially among the commonfolk, a vast majority of which all had dreams of their own and were deeply moved and inspired by Roger's adventures. Thus, to the better part of the world Roger was an idol and hero, one who had reached the zenith. There was no question of 'above'.
I don't have evidence but I am pretty sure 'Four Emperors' was propagated not by layfolk but rather by one of the authority figures (Marines?). That's why 'Pirate King' is a much more colloquial and endearing term than 'Four Emperors' which seems to be more bureaucratic, and why 'king' and 'emperor' are not measured on the same scale in this case. 'Four Emperors' does not evoke the same emotions in people's hearts that 'Pirate King (Gol D. Roger)' does!
It was not an issue of deciding who the next Pirate King was--the four did not decide to settle on a collective epithet instead of aiming to be the greatest among themselves, and more importantly, I don't think they gave themselves the title.