Number four's dynasty is Jannabid.
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Yeah, the ruler in question is Abu Tahir al-Jannabi, the leader of the Qarmatian state in Al Bahrein. Seeing that people here have beat me to identifying the six mystery rulers, I might as well post a more in-depth look at each of them.
* Judging by what we already know of the bookmark (taking place in 930s) and the young look of the character, the first character is most likely Otto I 'the Great', the king of Germany. When he was coronated the king of Germany after his father Henry the Fowler died of a stroke, he was only 24 years old, so the character's young look would work, and while the in-game portrait of Otto shows him at his date of death at the age of 61, his facial shape is still quite similar to his young self. According to the game files, Otto ruled from 936 to 973.
* Considering that the new bookmark has been hinted to take place in 930s, the second figure should be Caliph Al-Qaim, but the figure looks more akin to his grandson Caliph al-Muizz. However, given that in 936 al-Muizz was 4 years old, I can only assume that Al-Qaim's portrait has been adjusted with the addition of a new bookmark. According to the game files, Al-Qaim ruled from 934 to 946.
* The third character is probably King Æthelstan of England: while Æthelstan's current character portrait has a beard, he is the only one of the English monarchs from said time period to have blonde hair like the character we see here, so he's the most likely candidate. According to the game files, Æthelstan ruled between 927 and 939.
* The fourth character doesn't match any of the characters in history files, but is most likely Abu Tahir al-Jannabi, the leader of the Qarmatian state in Al Bahrein. The character's coat of arms are those of the in-game Jannabid dynasty, who in 867 bookmark hold the duchy of Al Bahrein, and looking at the character's portrait, we can clearly see that they're a Muslim duke, indicating that they're a duke rather than a king, like the other characters in the menu: and seeing that Snow Crystal had already teased the introduction of Qarmatian heresy before, it wouldn't be that big of a stretch to assume that Qarmatian heresy is going to be introduced with the new bookmark, which would explain why a mere duke is considered an "interesting character". According to Wikipedia, Abu Tahir ruled from 923 to 944.
* The fifth character is King Árpad Zoltán of Magyar: barring his hairline, the face seen in the image is identical to his portrait in Hungary's ruler history. According to the game files, Zoltan ruled from 907 to 955.
* The final character is most probably King Hakon, the first Christian king of Norway. Again, the current portrait of Hakon and the portrait presented here are very similar, and he is the only Norwegian king from the dynasty with a similar hair colour to begin with. According to the game files, Hakon ruled from 934 to 961.
All in all, while the hint does include even more material to suggest a new bookmark in the 930s, it doesn't actually help to narrow down the precise date any further: as I mentioned before, currently all the information we have about the bookmark would indicate tht the bookmark would take place between 936 and 939; and while the presence of Otto the Great helps to dispel any possibilities for the bookmark taking place prior to 936, the information we have doesn't enable us to narrow the date down any further.