You need 80 "devotion" points to get to level 4, same with taming and well every not hidden aura stat, they work the same as nature, determination, pride etc. 15 days may not be enough, as taming and devotion are slowest to raise. Of course items like shackles, these rings in accessory shop selled in 7-pack, crown of thorns and rules increase taming. Last time i played there was a way to raise taming instantly by 2 levels (One exception is sadist slaves). You have to rent dungeon, put your slave in cell, buy random cheap slave from market, switch them, then you make your slave guilty (level of guilt doesn't really matter), and then punish -> example of punishment -> execute. As for detailed info about your slave, i suggest to get the crown (i think?) from special mission in white town. I didn't see any changes in changelog that would suggest that devotion formula has been changed. Maybe some values has been altered. For maximum devotion You should set rules to "sleep in my bed" and " eat leftovers", and reward slave with spend time together, and sexual rewards each once per day (careful with massage, it tends to cause spoiling). Anyway i suspect You're talking about Isabella, yeah she can cause some troubles with her traits, but it's completely possible to make her devoted, she just takes more time.
I kinda regret writing so much details like formulas for devotion on wiki, it just take away exploration of consequences. Should have made just new page with some spoiler warning on optimal way to train a slave. Well, too bad i'm too lazy to make such action ;P.