a1=4/2=2
a2=8/4=2
a3=16/8=2
a4=32/16=2
vth=(a1+a2+a3+a4)/(1/2+ 1/4+1/8+1/16)=128/15
Rth=1/(1/2+ 1/4+1/8+1/16)=16/15
i=Vth/Rth+Rl=(128/15)/[(16/15)+1]=128/31