Mailbag: The Neandertal species question

Hello Dr. Hawks, I understand your a busy man so my question will be brief. I learned in my biology class that two different species can not interbreed and produce fertile offspring. If this is the case, how can we carry 4% Neanderthal genes? If they were a different species we should have not been able to crossbreed. Could you please explain this if you have time?

They weren’t a different species.