Optimum Resolution for Printing Photos

I'm going to get 150 of my photo files printed at Asda as I can get them
done for 5p each at that quantity. Most of my files are at 7 mpx i.e. quite
large. The prints are 6" x 4" therefore is it actually worth sinding these
large files or should I re-size to say 1280 x 960 (1.3 mpx) or whatever the
ratio is to give 6 x 4. Digital prints do show a lot of detail and I don't
want to lose that but I can't see all the detail on a 7 mpx file being
visible on such a small print