Answer-
The length of BC is 8 units.
Solution-
Mid-Point Theorem-
The line segment connecting the midpoints of two sides of a triangle is parallel to the third side and is congruent to one half of the third side.
As given,
AD = DB, so D is the mid point of AB
FA = FC, so F is the mid point of AC
Hence, DF is the line segment connecting the midpoints of two sides of a triangle.
Applying the theorem,
BC = 2DF = 2×4 = 8 units