Short answer: Must try both 8ohms and 4ohms taps, hear which one you like.
Long answer:
Output transformer is used to match tube with speaker impedance. Connect to the right tap can yield minimum distortion and maximum power transfer from tube to spk.
However, spk impedance is not constant across the whole frequency spectrum; a nominal 8ohms speaker may have impedance range from 4 to 10ohms. A nominal 6ohms spk may have impedance range from 2 to 8ohms and so on. So you never know which tap give min distortion and max power until you really hear the result, or use equipment to measure the actual output.
