A few factors come into play with thicker wood. Mainly, there is more area of the saw blade in the wood at one time, which, depending on the number of teeth, tooth angle and grind, gullet size and pitch, and horse power of the saw motor, can cause the saw to bog down. Even if you slow down your feed rate, the blade can heat up and warp, the saw motor can heat up, the wood can pinch or bind the blade....causing the saw motor to trip the heat sensor or a breaker in your electric panel, or if the saw has plenty of horse power, a kick back can occur, sending the board past you or through you at rocket speed. 8/4 wood can have some real stresses built up in the board, which, when ripped, can either pinch the saw cut together or open the saw cut up. It is the pinching shut that you need to watch. If the saw kerf closes as the board is being sawed, get ready for problems. I have found that stopping the saw and pounding a shim into the kerf to hold it open allows the cut to be made without pinching the blade. This requires shutting off the saw in the middle of the cut, adding the shim, then backing up the board to take the teeth out of the wood, then holding on for dear life and re-starting the saw and continuing the cut. Some woodworkers may scream and set off sirens when reading this, but in the case I am presenting, I have had to hold boards like wrestling an alligator until the cut is completed because the kerf pinched shut and the board wanted to fly out of the saw. Anti-kick back pawls and the spreader on your table saw guard are your protection from this safety issue. A lot of woodworkers take the guard off (including me), but the hazard I just described is exactly why these guards were invented.
There is a whole science to saw blade design. It is best to go to a saw blade manufacturer or talk to one on the phone and tell them what you are cutting. They will recommend the right blade for thick wood ripping. Here is one link to give you some info about saw blades. There is a lot to learn so study up on the terms and why saws blades work the way they do....
http://www.sharptool.com/PitchVsDepth.html
Here is another link. Look at this blade ... LM71Thick Stock Rip Blades at this link...
http://www.carbidespecialties.com/ripping_blades.htm
I would not recommend using thin kerf saw blades for ripping 8/4 wood. It is too easy for the thin kerf saw disk to warp when over heated...in my opinion.