I still think 4 feet would be too wide of a cut for a mill using tires but I will give you some numbers from my homemade tire mill that you can use to extrapolate to your mill.
To figure the contact area of the blade to the tires I will take 24"(the diameter of the tire)times pi(3.14) times 7/8" (the width of the blade in contact with the tire) Which gives me about 66 square inches of contact. That times the pressure in the tires (35 psi) gives me 2310 pounds of max stretch on the blade. But that stretch is divided by the top and bottom of the blade. So I have about 1155 pounds of strain on the blade which is well below the 1500 pounds that is the recommended minimum strain that should be on the blade.
I run my tires at 35 psi because if I go much above that the rubber of the tire deforms around the blade and takes the set out and bogs the engine. But if I had a separate way to tension the blade I could run 60psi in the blades then tension the blade until the rubber starts to deform I would be able to get 2000 pounds of strain on the blade.
Thanks for doing the math
I never thought about checking on mine to see what the tension is to stop the flutter. I'm running 1.25" blade so your 7/8" contact is about right. However, I'm using doughnut spare tires that run at 60 or 65 psi but they are a little smaller (22.5" dia). They are VERY stiff and do not deform much around the blade for as far as I've dared to push the blade tension. I just tighten until flutter is at a minimum. So with your calculations -> 22.5 x 3.14 x 7/8 x 60psi = 3,700 lbs of tension (max). But that tension is shared by the top and bottom sections of blade, so my actual blade tension would be half of that or 1,850 lbs. My (designed) max cut is 42". The 42" bed is just so I don't have to bibby down some monster logs. I don't plan on splitting anything that big. With 18hp gas and a sharp blade, I did a good job on some 30" logs.