Thanks to melipone, I now have the answer to my question. However, to clarify for anyone else who sees this, I want to point out that enumerate() is not germane to the question and is not necessary. Once the worksheets have been created like this:
worksheet_names = ['a', 'b', 'c', 'd']
for name in worksheet_names:
wb.add_sheet(name)
It is possible to write to any of the worksheets in any order you want, with any data you want. All you need to know is the index position of the worksheet and it can be accessed with "ws = wb.get_sheet(index_position)". For example:
dataset = ['100', '200', '300', '400']
ws = wb.get_sheet(2)
ws.write(1, 1, dataset[2])
ws.write(2, 1, "This is something else I want to write in the third worksheet")
ws = wb.get_sheet(0)
ws.write(1, 1, "Cell 1, 1 contains data from a dict")
ws.write(2, 1, "This is something else I want to write in the first worksheet")
ws = wb.get_sheet(1)
ws.write(1, 1, "The data in this cell comes from a different dict")
ws.write(2, 1, "Write this in the second worksheet")
ws = wb.get_sheet(3)
ws.write(1, 1, "This is the 4th worksheet")