I think 5 & 6 are unethical, but 4 also smells a little fishy. But, when you think about the fact that lots of us use mileage cards for company expenses (or even entertain for work at Idine restaurants to get miles through AAdvantage or Mileage Plus Dining), that is sort of like 4. And, since I don't think anyone would say that paying for expenses you're going to get reimbursed for in the manner that happens to earn you miles is unethical (assuming, of course that you don't intentionally take an important client to a hideously overpriced, terrible restaurant solely to rack up miles), I think 4 is fine.