To find the unit rate of minutes spent in yoga class each day, you need to divide the total minutes by the number of days.
Bella spent a total of 128 minutes in yoga class over 4 days.
So, the calculation is:
\[ \text{Minutes per day} = \frac{\text{Total minutes}}{\text{Number of days}} = \frac{128 \text{ minutes}}{4 \text{ days}} = 32 \text{ minutes per day} \]
Therefore, Bella spent 32 minutes in yoga class each day.