Generally yamaha suggests the first service at 20 hours. Then again at 50 hours for the second, where they go completely through the machine, adjust valves,etc. I haven't heard of the 4 or 5 tankful method, LOL. This is my third Yamaha I bought new and from two different dealers, they both told me 20 hours for the first, then again at 50. As for riding your new machine hard when new, I personally wouldn't go to crazy on the throttle just yet. I was told by a very good Yamaha mechanic, that the valves and different components need to seat properly and wear in. I guess it's just like a new car, you don't go throttle happy until the engine breaks in correctly. When all three of my quads were new I wouldn't give it throttle more then halfway and if by chance I had to it wasn't for very long, only in short spurts. There has been much contraversey over the "break-in period", some swear by ride it the way you would normaly ride it, and some say go easy until all the parts wear in. I chose the latter and I have never had any issues whatsoever with engine problems. But hey you bought it and you decide right?
Bluegrizz is absolutely correct on the gas issue. If you go easy on the throttle a tankful will last quite a while. If you ride in 4X4 mode alot the gas will get sucked up rather fast. Where I am from here in New England we get snow and it gets deep at times where 4X4 is the only choice. The fuel seems to get used up rather fast getting from point A to B as where in 2 wheel mode the same distance hardly any is consumed. As Bluegrizz says,"if you stay on the gas you will use fuel, but that's what it's all about." Good luck and congrats on your new machine.