The area of a square road around a $4$-m-wide road inside it equals $1$ hectare. The side of the field (outer) is —

With outer side $x$ and a $4$ m wide road, the inner side is $x-8$, so the road's area is $x^2-(x-8)^2=16x-64$. Setting this equal to $1$ hectare $=10000$ sq m gives $16x=10064$, so $x=629$ m.