I need to copy data in one sheet to another. The data to be copied is
of different sizes, may be in different locations on the source sheet
and has varying strings that define the top-left and bottom-right cells
depending on the situation. And the sheet names will also vary.

To simplify the problem, I have removed the variables and have inserted
number in the cell ranges. I need to understand why this works:

Sheets(2).Range("A1").Value = Sheets(1).Range("A1:D1").Value

and this produces a '1004' Application defined or Object defined error

Sheets(2).Range(cells(1,1)).Value = Sheets(1).Range(cells(1,1),
cells(1,4)).Value

I have also tried:

Sheets(1).Select
Sheets(1).Range(Cells(1, 1), Cells(1, 4)).Select
Selection.Copy
Sheets(2).Select
Range(Cells(1, 1)).Select
ActiveSheet.Pastevalue

But this produces a 1004 error at the line "Range(Cells(1, 1)).Select"