To be more specific than your post implies, you want the your impedance switch set to 4 ohms when you remove two power tubes if it is normally set to 8 ohms with all four. The implication being that you should play the amp into an 8 ohm load with the amps impedance switch set to four ohms, effectively doubling the speaker load impedance. The article probably did specify this.
To be clear, if you didn't have an impedance switch and your amp wants an impedance of 8 ohms, if you pull two power tubes you want to plug into a 16 ohm load, effectively doubling the speaker load impedance. This is probably what you were thinking.