75km

Distance between second and fourth house = 40 km

Distance between the second and third house + Distance between the third and fourth house = 40 km.

Distance between the second and third house + (Distance between the second and third house - 10) = 40 km.

Therefore,

2 * Distance between the second and third house = 50 km

=> Distance between the second and third house = 25 km

Hence, distance between the third and fourth house = 15 km

Distance between the first and fourth village = 60 + 15 = 75 km