You are Generation 1, so Generation 4 are your great grandparents of whom you have 8. But your great grandparents may not have been 100% Scots-Irish or any other ethnicity, so these reports are not worth much. These results are based on the database that they maintain and only provide a some probability that your ancestors fit into this population. This is a marketing parlor trick. It may be interesting and even fun, but it’s not a serious tool.
If you know the names of your grandparents, then you can probably find enough ancestors to answer your question. Companies like Ancestry are selling their subscriptions and the DNA report is very useful to them for selling subscriptions. Some of this is free and you may be able to get everything you are looking for without paying for information.
Depends on how pure an ancestor was. As humans moved around in recent centuries, DNA mixed.