Rookie at this myself.Right at 4lbs. is recommended. I do think that pure trail riding you will have better traction at 4. If you drive on grass, such as your yard, higher pressure causes less damage. I think I am currently running 6lbs in both our four wheelers. I suggest you try that and see what you think. You will have a softer ride and more traction compared to 12lbs. My granddaughter's four wheeler I have all the way down to 3.5lbs to improve the teeth jarring ride on her little Chinese 110.
Have not owned four wheelers long enough to form an opinion about tires much. Ones I have been thinking about somewhat are the Carlisle Badlands XTR. They are radials, 3 ply, and fairly light. They also seem a little expensive. You would need the 205/80 and 255/65 to up one size. I would be careful about 6 ply or 8 ply tires unless you are really concerned about punctures. They are considerably heavier tires. I man a couple of hours from me is selling a nearly new set for a good discount, wants a more swamp, heavy lugged mud tire. I am seriously considering making the ride if he would meet me part way next week. At the same time I can't say the Kenda tires they come with are bad. Kind of prefer them over the Maxxis that come on the Yamaha.