Make sure only one page is highlighted in the spread.
In the pages panel, make sure only the left (or right) page is highlighted when you change the number of columns. Sometimes both sides of the spread will be selected which is why both will have the column number changed.
Alternatively, if this is consistent across your document, add the columns into your master page so you only need to make the change once.
Thanks guys, I did that and there is no value in the column number field. I type in 4 and it changes one page. I then select the facing page and in Columns and Margins the column value field has a number (say 5) and if I type anything else in that field it changes both pages...
News Flash! I pulled the Pages menu away from the dock (so it doesn't close when I select Margins and Columns) and selected a page and that works.
Thanks for the replies!
Use master pages, or just create two independent text boxes; one with 4 cols, the other with 5.