Protecting own land from neighbours

Actually we've averagely around 3 feet in our backside. But the neighbour has a blind luck. Because his parent document had an issue & the original first owner of the land paid the penultimate(the one who sold the land to the current problematic neighbour) owner of the land for 4 feets & verbally told him to disvow 4 feets. unfortunately, there is no paper record for that 4 feet. Which became a problem for us, now the neighbour who have that extra 4 feet, wants to take our backside land for that 4 feet. Also states that our 3feet land is on th roadside. Now we want to know how to protect our land. If we go for the court to survey & what if the surveyor measures against us(actually we've 3feet infront of the building & 3 feet at the backside) In the meantime that neighbour regularly disturb us by doing some work at that disputed backside land. How to stop him. Shall we build a wall. what if he tries to stop us while building that wall. What can be a permanent solution.