Sorry, the shown link above was irrelevant. As the explanation of batch_size says in link 1, the batch size of all GPUs on the current node should be specified, so given batch_size here should be 64 under 4 GPUs environment for 256 batch size per iteration.
I may not be clear about your second question.
Assuming 1 GPU per node and 4GPUs environment, each process consumes len(dataset) / 4 amount of inputs per epoch. Then if the total batch size is 256, each process takes care of 64 batch per iteration (this 64 should be specified as the argument). Even if  you specify batch_size as 64, the amount of inputs to be processed for each iteration is 256 and total iteration per epoch is len(dataset) / 256.