The formulas for degrees of freedom are summarized below:

df Subjects = N - 1 = 4 - 1 = 3

df Conditions = a - 1 = 2 - 1 = 1

df error = (N -1)(a-1) = 3

df total = aN - 1 = 2 x 4 -1 = 8-1 = 7

a is the number of conditions; N is the total number of subjects.

The error term in within-subject ANOVA designs is equal to the Subjects x Condition interaction. As such, it is a measure of the degree to which the effect of conditions is different for different subjects. Remember that when variables interact, the effect of one variable differs depending on the level of another variable. The error term is a measure of the degree to which the effect of the variable "Condition" is different for different levels of the variable "Subjects." (Each subject is a different level of the variable "Subjects.") Low interaction (low error) means that the effect of conditions is consistent across subjects. High interaction (high error) means that the effect of conditions is not consistent across "subjects."