Im still trying to learn about the 4 stroke motors. Youd laugh if you knew how long it took me to figure out how to change the oil in the thing the first time, I know virtually nothing about them. I can rebuild a 2 stroke motor with my eyes closed, top to bottom, but the thumper motors just scare the crap outta me so I always just drop it off at the shop or my buddies house and he does it on the side on a sunday when the shop is closed. I sold my last one for that very reason and stuck to the familiar 2 stroke motors. How often do you recommend checking the valves? I think someone told me 5 hours. I checked them every 10 on my last yz450f.
I just rebuilt the topend on my 09. New piston/rings and valve seals. To be honest I was intimidated at first but it was really easy. (I've only rebuilt 3 two stroke top ends and 1 bottom end prior to this)The only hard part was keeping all the rings in place. The manual was very helpful though.
As far as checking valves go, I usually check them like every 20hrs cause most of the time they dont move if you keep your airfilter clean. And when the valves do get tight its pretty easy to tell. Hard starting when the motor is hot and the motor just doesnt sound right or run smoothly. Another thing to remember is that before you check your valves make sure your bike is clean. The last thing you want is dirt falling into the head causing all sorts of wear.