Jehovah said, Go through the middle of the city, through the middle of Jerusalem, and make a sign on (or designate) the foreheads of the men (vir) who groan and sigh over all the abominations. Ezek 9:4.

Here 'designating foreheads' does not mean putting a sign or stroke on their foreheads but a way of distinguishing from others. Something similar is said in John about men who did not have God's sign on their foreheads being subject to condemnation, Rev 9:4. Here again 'having a sign' stands for a way of distinguishing.

[2] In the same book this sign is also called 'a mark', where reference is made to placing a mark on the hand and on the forehead, Rev 14:9. That which was meant by signs and marks the Jewish Church represented by binding the first and great commandment on to the hand and on to the forehead, a practice mentioned in Moses,

Hear, O Israel, Jehovah our God is one Jehovah; you shall love Jehovah your God, with all your heart, and with all your soul, and with all your might. And you shall bind them as a sign on to your hand; and let them be as frontlets between your eyes. Deut 6:4, 5, 8; 11:13, 18.

This represented the requirement to distinguish the commandment concerning love above all other commandments. This shows what making a sign on the hand and on the forehead means.

In Isaiah,

One is coming to gather all nations and tongues, and they will come and see My glory, and I will set a sign among them. Isa 66:18, 19.

And in David,

Look to me and have compassion on me; give Your strength to Your servant, and save the son of Your handmaid. Make a sign in me for what is good, and let those who hate me see and be put to shame. Ps 86:16, 17.

From all these quotations it is now clear what 'a sign' means. Let nobody suppose therefore that some sign was put on a man called Cain, for the internal sense of the Word embodies matters altogether different from those of the sense of the letter.