=sum(index($b$10000:index($b$2:$b$10000,large(index(($b$2:$b$10000<>0)*row($b$1:$b$9999),0),4)),0)) last 4
=sum(index($b$10000:index($b$2:$b$10000,large(index(($b$2:$b$10000<>0)*row($b$1:$b$9999),0),8)),0)) last 8