i'm not familiar with the dimensions of A4 but if you are, my brute force
and ignorance would tell me to squeeze and set fonts to allow you to reduce
the page width down to it.

instructions to change page width:
go into design view,
move data fields to the location you want (moving them from the scaled area
into grey area will auto increase the width)
reduce the page width by clicking on the right page boarder and dragging to
the left. it wont let you over reduce, meaning, it will stop at the farthest
right field.

Peter,

In design view of your report, under the File|Page Layout menu, you will
see the Laft and Right margin settings for the report.  You can alter
these if you like.

Then, look at the Ruler along the top of the report design, and see how
wide it is to the right edge of the report design area.  Either that or
look at the Width property of the Detail section of the report.

Now do some maths...  Add the report design width plus the left and
right margins.  If it's more than 21cm, you need to change your layout.