The mark in Eze. 9:4 is a mark placed on the forehead and is a sign of "exemption and or judgement". Like a signature written on the foreheads of those who morn because of the down fall of Jerusalem.
Eze 9:4 And the LORD said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof.
The "mark" spoken of in Eze. 9:4 is not the same mark as the mark of the beast.
All of Eze. chapter 9 is about God's judgement against those in Jersusalem who have been lawless of God's law and the man in linen with the inkhorn is the man who writes something into the foreheads of those in Jerusalem who are obedient unto God.
Eze 9:2 And, behold, six men came from the way of the higher gate, which lieth toward the north, and every man a slaughter weapon in his hand; and one man among them was clothed with linen, with a writer's inkhorn by his side: and they went in, and stood beside the brasen altar.
So what is happening here is that happening here is that God is judging those who rebel and commit paganism in the temple and the five men of the six who were called are ordered to go through and kill everybody who is guilty of iniquity. But God is a loving and faithful God so the man in the Linen write something into the forehead of those who mourn so the mark will be seen by the men with the slaughter weapons and they are past by and not killed.
Eze 9:3 And the glory of the God of Israel was gone up from the cherub, whereupon he was, to the threshold of the house. And he called to the man clothed with linen, which had the writer's inkhorn by his side;
Eze 9:4 And the LORD said unto him, Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that be done in the midst thereof.
Eze 9:5 And to the others he said in mine hearing, Go ye after him through the city, and smite: let not your eye spare, neither have ye pity:
Eze 9:6 Slay utterly old and young, both maids, and little children, and women: but come not near any man upon whom is the mark; and begin at my sanctuary. Then they began at the ancient men which were before the house.
I have always believed that the man in linen is Christ
Eze 9:11 And, behold, the man clothed with linen, which had the inkhorn by his side, reported the matter, saying, I have done as thou hast commanded me.
This is a great example of how God doesn't always remove believers from out of the trouble but instead He left them in the middle of the trouble but provided protection from the men with slaughter weapons. IE A signiture of protection given by God. Not the mark of the beast that will be given by men not for the purpose of protection but for the purpose of destruction.