how many yards of cement do I need for a slab of 6x8 4"tick

The short answer is .6 of a yard. A yard of concrete contains 27 cubic feet of concrete (3 ft by 3 ft by 3 ft ) Your slab is 48 square feet and 4 inches or 1/3 of a foot deep. Multiple 48 by 1/3 and you get 16 cubic feet, then divide that by 27 and your get .6. Your mininium order would be 1 yard if your having it delivered by truck already mixed. The golden rule of concrete is always order more than you think you need. Coming up short will cost you at least 3 times the amount you might waste. I try and make simple forms for stepping stones, or pier blocks or something like that for extra concrete. A small order of just one yard will be pretty expensive on a per yard basis. I may be cheaper (but a lot more work) to rent a mixer and buy bags and mix on sight. The bags will usually indicated the amount of concrete in square feet, of which you need 16.