i've packed for four days in a nice expandable backpack i have. and my general rule is if your gona be gone more than four days to find a place to wash your stuff so you don't have to pack more.
but then again i live light when i travel so i'm not gonna say everyone i know can pack for four days in a backpack.
the key here is to buy a backpack that is self condensing so if its empty its thin and has side straps to keep it thin, but at the same time can expand enough to suit your means. it also needs to have a padded back and nice shoulder straps so its comfy.
i like the http://www.amazon.com/gp/product/B002SXMRO4/sr=8-1/qid=1310928239/ref=olp_product_details?ie=UTF8&me=&qid=1310928239&sr=8-1&seller= Backpacks myself (that isnt a pick of mine i have had mine for a long time but it is a comparable model)
and they last, i got mine four years ago and i still use it on a daily basis.
if you buy any SwissGear bags keep the receipt because the bags have a lifetime warranty, receipt required.
2009 Street Triple 675 R
12,000 miles and some-days it still scares me.