There is a lot of good information out on the Internet about registering a
homebuilt aircraft. This page discusses aspects that are specific to the BD-4.
When you purchase a flying homebuilt you need to check the Operation Limitations
to see how this specific aircraft is allowed to be used. Older versions of this
paper often forbid flying over densely populated areas. Later versions also have
a more flexible handling of changes you may want to implement.
To give you a better idea of these Operation Limitations, here
is a copy of the ones from N236CP (Colin Powers). Or download a PDF
document with the legal papers of this plane.
It would be nice to compare this sample with a later version. If you can take a
copy of a later version of the Operation Limitations please send them my
way and I will present them here.