Sorry to hear about your first game. A cavalier would be most welcome. But you arrive at quite a delicate time.
The party is very close to the culmination of the first book and the start of the next. They are preparing for an attack on the Stag Lord's fort in the wilderness and ending the hold of bandits in the region.
I would be fine if you started at level 4 since everyone else has already surpassed it and you will soon have downtime to earn xp and catch up without going on a specific quest.
As such, your backstory should include a hint of how he reached that level. You could have recieved the exploration charter if you want and had its delivery delayed, or your character was busy, etc. Or you can have other motivations as long as they fit with the theme of the campaign.
I hope you can get a character up really soon and eagerly wait.