1280*1024 has a ratio of 5:4 whille 640*480 has a ratio of 4:3,
so yes if u resize from the bigger to the smaller the picture will
look slight squashed sideways.
4R size is 6" by 4" which is a ratio of 3:2, so NONE of the above
resolutions are good ahahah, if u give them the pics in those
resolutions, they will either crop off to 3:2 ratio, or print until
there'll be white gaps, where they'll fit the longest side onto the
paper, then u crap the white parts off urself.
so, if i were to print 4R i will resize to a 3:2 ratio and make the
size 6"x4". hence i will make the picture 1800x1200 becos i
want/like 300dpi. those figures will gimme exactly 6"x4"
however u don't have to use such a high resolution, anything around
150dpi is still very good. just give them the highest then u can
afford. that should be ok.