Suppose the Excel column is called "mycolumn"

Then in your Word document, you probably have a field like

{ MERGEFIELD mycolumn }

(use Alt-F9 to show the {} )

If so, select that, then press Ctrl-F9 to give you

{ { MERGEFIELD mycolumn } }

then type a "=" and "* 1.04" around that { MERGEFIELD mycolumn } to give you
something like

{ ={ MERGEFIELD mycolumn } * 1.04 }

[NB, each pair of {} is a special pair of "field braces" that you can insert
using ctrl-F9.]

Then try again...