OA=OC=√2 /2 *AB=2 √2
∴AE=√2
又△AEF∽△CEB
∴AF/BC=AE/EC,即
AF=AE*BC/(OE+OC)
=√2* 4/(√2+2√2)
=4/3
∴DF=AD-AF=4-4/3
=8/3