You are setting
comblist to a list of strings. If you want to iterate over comboboxes you need to iterate over the comboboxes, not strings.
There are many ways to do this. In your specific case it’s probably easiest to just get rid of the original definition of
comblist and replace it with a list of comboxes.
comblist =  for i in range(4): comb= ttk. Combobox(root, values=value1, state="readonly") comblist.append(comb) ...
With that, your code to iterate over
comblist will work:
for cmb1 in comblist: cmb1.set("None")
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