Thank you for your post.
As you noticed, fuzzylite v4.0 has an issue with Trapezoids when a=b or c=d. The same occurs with Triangles when a=b or b=c. The solution is exactly what you mention. I have addressed these details in version 5.0, which I expect to release the first week of August.
In order to fix these issues, you will need to recompile the library such that Eclipse takes the modifications.