Thevenin equivalent and maximum power transfer theorem question
You are correct. R4 must be taken into account when calculating the Thevenin resistance. The Thevenin resistance is defined as the resistance looking into the circuit from the load terminals.  It is clear that R4 is in series with the rest of the circuit. Imagine if R4 was infinite. According to your classmate, it would have no effect on the Thevenin resistance when it is clear that it makes the resistance infinite. Don't rely on your classmates but use the knowledge that you have gained in the classroom. That is why you are there.