You will have to be a little more specific, year time frame? type of Harley? etc.....
I can't think of anything off hand with being limited to only 4 with the POSSIBLE exception of how many many bikes they made the first year...only 3 were built....not sure about 1904, but only 11 were made in 1905...the numbers SLOWLY increased from there.
as far as limited edition bikes, or concept bikes...I have never heard of any only limited to 4....if I had some more info to work with...I might be able to help answer your question.
