Probleam with combo box list item which connect to table in database

It seems that you are not handling the change event properly.
From your description of the problem above, please try to make sure the following points.

1. All the four combo boxes have different names in context of their Identity and Event Handlers (Selected Index Changed)

2. Make sure that you are not performing any inter-connected selection index changing process while selecting items from any combo box. What does this mean is when you handle the selected index changed event for the combo box 1, it does not have anything which changes combo box 2, 3 and 4

To achieve your goal;
What I would do is;

1. Load all the combo boxes with the item names from the table.

2. Keep the item names (DataTable) in either a session variable or a global variable

3. On selected index changed property of combo box 1, I load the combo box 2, 3, 4 with all the items from the session or global variable, then iterate through all the items and remove the item which is selected in combo box 1

4. I would do the step 3 for combo box 2 and 3

This way, all the items would be different in combo box 1, 2 3 and 4

Happy coding :)