As spelled out in
Maximum Advantage Level (
Adventurers, p. 46):
When an advantage comes in levels or varieties, the [...] the number of levels allowed [is] the highest of these three limits:- The maximum noted on your professional template.
- The maximum under Exceptional Abilities (Exploits, p. 93).
- The maximum noted in the advantage description itself.
So since the knight template notes six levels, and that's higher than the four levels in the advantage description, knights can have six levels.