Hmmm. OK I think I got it.
What I would do is ask the first person: "Would the NEXT person say that door number one is the one we want if I ask him that?" And whatever the answer I get , I simply reverse it.
Basically if the man says "YES" then I should consider it as "NO", and if it's a "NO" I should take it as "YES". Reason being that if the person I ask is truthful, he will think regarding the lie that the next person is going to tell me and truthfully tells me the liar's answer, which is something I want to go against (For example if door number one is safe, the truthful man knows the liar will say it's NOT safe, and he will say "No" to me, so if I reverse it then it is "Yes" which is correct since it is safe). If the person I am talking to is a liar, he will think about the truthful answer the next man will tell me, and then HE will lie about it when telling me, so again the correct answer is the opposite of what is being said (If the door is safe, the liar knows the truthful man will say it is safe, but because he is a liar he will say it is not safe, so if I reverse it then it is "Yes" which is correct again)
By this method I establish a correct answer every question. I just ask them one by one regarding different doors one by one, and will know whether that door is safe or not every time.
The only remaining obstacle here is that we get to ask four questions, and there are five doors. I think the answer to that lies in the fact that if after 4 questions I get three yeses and one no, the door that I was told about to be a "no" is the safe one that I should use (after reversing the answer), and I don't need to know any other door. If the four doors I ask about are all yeses, then in reality they are all "no", and therefore the only remaining door is the truly safe one.
Did I get it right?