Lenovo x230 starts under 3.5 pounds, good battery's life, i7 available, up to 16gb. the old x220 had a great keyboarf but the x230 uses a different keyboard, dunno if better or worse(could probably get the x220 cheap somewhere)
with 9cell and slice battery's(additional weight), up to 15 hours
though last I checked, the i7variant is only dual core