Was just reading over some wording, and came to the following RAW conclusions:
1)A ninja with monk levels doesn't need 4 monk levels in order to count monk levels towards a ki pool.
2)A ninja that wants to use wisdom as the ki determening stat needs at least 4 monk levels (or 3 if ki mystic archtype).
Explanation
1) the wording specifies classes that give ki pools, without specifying that you have to be far enough in that class to have gotten the ki pool. Compared to Improved Uncanny Dodge, where the wording specifically states that you need Uncanny Dodge from a different class in order to count those class levels.
2)"The choice of which score is used is determined when the second class ability is gained".
While they are subtle points, resulting in minor bonuses of ki when splashing monk, even 1 or 2 points of ki can make a difference.