The 4-gram BLEU score is broadly accepted as the evaluate standard when we tune the other parameters using the minimum error rate training, which means that the MERT stage will not stop until it reaches the highest 4-gram BLEU on the development set.
the same sentence  To evaluate this hypothesis, the alignment model is set the same as the best performance in Table 3, and 5-gram LM with improved KN smoothing is set for LM.