I think the sentence is open to interpretation, but my first reaction to the statement would be that:
There are only four evaluation sessions given throughout the year. Each session has multiple slots, but these slots are determined well in advance.
Without more information, I cannot say for sure, but it is possible/likely that each evaluation takes place over the course of one day, as you noted (so there would be four days of evaluations).
I think it is highly *unlikely* that only four people are evaluated throughout the entire year (otherwise there would not be multiple time slots).