The four horns are the powers which have scattered Judah, but ‘these are come to fray them.’ You could translate that ‘to terrorise them’, the Hebrew means to cause them to tremble.
Christians, engaged in witness are all craftsmen. We must not be intimidated by worldly powers, for God is teaching us that though they are not aware of it and it takes them totally by surprise, he is already constructing the means by which they will be brought down. They are simple means, means that they would not have regarded as being of any threat to them, and yet they will fall.