Do you look at how the props are rotating in the water on the outside to know if they force upwards or downwards forces on the stern? Seems like on the insides, the wash from both props would give no bite. Because if you look at the direction they rotate, the blade coming out of the water will want to pull the stern down. But while there is a blade coming out, there is another one going in, wanting to lift the stern up. Which has more influence?

On my F311 SR1, turning in wouldnt hold the bow and lifted the stern. I had to turn them out. This was with 3 blade mirages and a higher than stock x-dim. ??

turning the props out always lifts the stern.
Turning them in always gives more bow lift.
Cleaver type props cause stern lift and round ear props give better bow lift.

And, then when you throw 4 blades into the mix??

One blade, ten blades, it's just more blades.

The blade STYLE (cleaver and chopper are the two most popular templates for a prop blade) is the main factor that influences bow or stern lift. Individual blade tweaks can influence these characteristics but to a lesser degree.

Now, there are stern lifting 4 blades and bow lifting 4 blades. Choose your poison.

We could go on to talk about 5 blades, too, but it is the same story.

Setback and X height are setup parameters that affect the choice regarding how many blades you need.

The higher the prop runs out of the water, the less blade area it has in the water at any given time. Less blade area is less load carrying ability. On a light boat it doesn't matter as much. On a heavy boat, a 3 blade may not be able to plane satisfactorily while a 4 blade may give it enough area to move the load.

Also, the more blades you have in a high X application, the less steering torque you must fight (a non issue in a twin but a VERY significant issue in a single).

In a raised X boat.
The closer that you run the prop shaft to the running surface of the water the more it will tend to lift the stern.