Welcome to the Board!
A piece of advice - don't used merged cells, they are an atrocity that cause all sorts of issues with things like sorting and VBA.
Use the "Center Across Selection" formatting option, as described here:
Tom’s Tutorials For Excel: Using Center Across Selection Instead of Merging Cells – Tom Urtis
So, regarding the VBA code, how exactly do you want this to work?
Are you going to be pre-selecting the rows that you want to apply it to?
Are you going to be selecting columns A-D, or do you want it to do that automatically for the rows you have pre-selected?
Or is there some logic that can be used to dynamically determine which rows to apply this to?