wouldnt say dangerous, but he different treds will make the car pull one way and give very poor results... Why would you want to buy second hand tyres? you would be better off just changin the front 2 (if you want good drive grip) or the rear 2 (if you dont want the back end to skip out, this is supposed to be the safer option aswell iv recently been told...) and then replacing the other 2 when you have a bit more money... OR buy re-tredded tyres...