We read in Teshuvot haRashba Meyuchas LehaRamban No. 284:
(….) “And also in regard to the words of the Rambam (Rabbi Moshe ben Maimon), of Blessed Memory, they are not intended to forbid the pilegesh to a commoner and permit it to a king. Rather, this is what he said: “And anyone who has relations with a woman “leshem zenut”, without Kiddushin, is whipped, because he has had sex with a kedesha ; and “leshem zenut” means he contacted and engaged in sex with her and did not render the relationship exclusive to him for the purpose of establishing a pilashut form of relationship, which thus rendered her a kedesha. And Rabbi Moshe of Blessed Memory did not say that anyone who has sex (simply) without Kiddushin is flogged.
And likewise, in his Laws of Kings, when he mentions the issue of pilegesh as it pertains to a king, he does not at all mention that its acceptability applies solely to a king.”
Now, in Laws of Kings 4: 4, Rambam clearly states that pilegesh is forbidden to a commoner: on what grounds then does Ramban say it isn't? I can't understand why. Yaavetz, in his famous response on pilegesh, confines himself to speculating that Ramban was in possession of a different text of the Mishneh Torah, in which the aforementioned prohibition was not there.
Has this topic been investigated by Jewish scholars?