For matched transistors, the current through each transistor is related to the input current by the current gain \( \beta \). The current \( I_{O4} \) can be calculated using the following formula:
\[
I_{O4} = \beta \cdot I_{in}
\]
where \( I_{in} = 1.25 \, \text{mA} \). Substituting the values:
\[
I_{O4} = 20 \times 1.25 \, \text{mA} = 25 \, \text{mA}.
\]
Thus, \( I_{O4} = 25 \, \text{mA} \).