How to be sure which sheet the code refers to

I am having great difficulty with this code:-

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I have been getting an error with Line 10 when line 5 was simply

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so after a little research I took advice and dimmed and set Workbook and Worksheet.
THen the code as presented did seem to work, at least it was not throwing error messages.
However nothing turned up at Sheet4 C1.
Looking further I found that the code Copied the relevant range on sheet4 (Which is empty) and therefore pasted nothing at C1.
I cant understand how my code has told it to do that! Please help
John

Thank you all for your interest and help. Essentially you all gave me the same answer, which was encouraging. Any way I have incorporated it into my code and it works.
I have a related hole in my knowledge which has come up in your replies. This is when to use Range(Cells and when to use Range(.Cells I frequently got an error referring to this when struggling with this bit of code.
John

I Thanks for that. I get the feeling that there is a big black hole in my understanding of the fundamentals of vba/Excel.
For instance after I had seen your post I carried on with the next part of the project withy the following code:-

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Of the three versions only that with line 30 works. The other two give error 1004 Application or Object defined error
Yet this seems to be exactly the setup that you were describing where 20 should have worked.
John