It sounds like you may not have set up your Vlookup formula correctly to reference the dropdown box. Here are the steps you can follow to set up Vlookup with validation:

1. Set up your dropdown box with the four items you want to choose from.
2. In the cell where you want the Vlookup result to appear, type in the Vlookup formula. For example, if you want the bank account number to appear in cell B2, your formula might look like this:
   Formula:

   =VLOOKUP(A2,Table1,2,FALSE) 

3. In the formula, A2 should be the cell reference for your dropdown box. Table1 should be the range of cells where your bank names and account numbers are stored. 2 should be the column number where your account numbers are located (assuming bank names are in column 1). FALSE should be included to ensure an exact match is found.
4. Once you have entered the formula, press Enter to see the result.
5. Test the formula by selecting each item in the dropdown box and verifying that the correct account number is displayed.

If you are still having trouble, double-check that your dropdown box and Vlookup formula are referencing the correct cells and ranges. It's also possible that there may be an error in your data that is causing the formula to pull the wrong information.