I'm starting a the Crazy Eights pattern and it tells me to make the cutting go faster, to stack my fat quarters 4 thick then cut. I notice that when I do that, my squares/patches are not accurate...am I doing something wrong?

I have never mastered that stack cutting. I usually stick with 2. I'm usually not in any hurry. I enjoy the whole process:)

Need to change the blade your rotary cutter maybe? I stack em really high sometimes but I change my blade often.

I've seen this advice in patterns also, but, for me, I know that I can not be accurate of I stack more than a double thickness. And I've found that cutting accurately is very important to the outcome of a nice block with seams matching and points not getting lost in seam allowance. I'm sure there are quilters that can cut 4 or more thicknesses and stay accurate, but it doesn't work for me.

In my experience I need a the larger rotary cutter and a sharp blade. Wear a cut proof glove, press down firmly on the ruler and make sure your perpendicular to the ruler (not slanting).

I actually have a new blade on my cutter, I changed the blade right before I started cutting. After the first few cuts, I started cutting my strips just a tad bigger so I can just trim the pieces down.

To cut 4 layers you need a larger rotary cutter and a new blade and they cut just fine.

I use the biggest rotary cutter for more than 2 layers. I think it's 60 mm.

I've stacked and it works great, but if you make a bad cut you've made more than one mistake. I've had to change my plans because I was distracted and cut wrong.

Press the first piece of fabric, stack the second one on top and then press it, the third and fourth one too.
Pressing them in this manner seems to make them stick to each other, and can make cutting through a stack more accurate. :D:D:D