Hi....
When the 6 year old has a tantrum, just pick up the 4 year old and take him out of the situation. Make sure that at those moments. he chooses his own clothes etc.
I think your 6 year old is trying to make his presence felt.... and he is trying to get his own way. While he is succeeding, he will continue. What happens if you intervene... and play with the 2 of them? Then you can ensure both can express their opinion.
I am not an expert.... but it sounds like no. 1 son feels that no. 2 son is in his space.... and he is trying to establish himself.
My middle child used to be physically hurt by his little sister. He never stood up to her. That would never happen now....He has grown in confidence. She has learned to keep out of his business!!!