Have you considered the ULA catalyst? I don't own one but everyone who owns one swears by them as far as I can tell. They are very popular among thru hikers and they are made of pretty tough stuff apparently. It's 75 liters for under 3 pounds (43 ounces). You were only hoping to get under 4 pounds. I think that ought to be easy for only 70 liter capacity. (That's why I have so little patience for these Kelty packs that have only 3000 cubic inches for over 4 and a half pounds.)