Power Query - 4 tables into one master file

So, you ended up with two columns for the date of birth (DOB and DOB1). Make sure both have the correct Date data type. Assuming that you do not have dates in both columns on any of the rows, now select both columns in the PQ Editor "Transform" or "Add Column" tab select 'Merge columns'. This will either replace the two separate columns by one (transform) of create a new column (add). Set the Date data type again for the merged column.

If this is not what you had in mind, please provide som example data. Attach a file or provide a link that gives access to it on OneDrive, Dropbox or similar.