Board Thread:Tapped Out Help/@comment-5.87.144.114-20170423090359/@comment-26561761-20170423232939

It doesn't seems like you're on part 4. It seems more likely on part 5 or 6.

If part 5, then you need to complete the second objective Make Homer Not Have To Pick Up Trash. If part 6, then the Dilapidated Rail Yard should be already available to purchase. Once placed and completed, the Dilapidated Rail Yard Prizes prize track should be enabled and you just need to turn in projects by tapping the Dilapidated Rail Yard.