I am building a horizontal fence with 4x4 posts and 1x4 redwood rails. I want to hide the fasteners. So I thought I could use pocket hole joinery and screw in from the side of the 4x4 into the redwood boards. But this seems to be an odd combination and I can't find any guidance on how to setup my Kreg jig. I suppose this is a "face to face" joint.
Shear strength of pocket holes isn't that great, I would avoid using pocket hole joinery for fencing.
The pocket holes failed pretty regularly around 100lbs of shear load (of course this depends on the wood tested). The wood began to open up around half that force.
Ideally if you are looking for hidden joints you would use a mortise and tenon style joinery. These are more difficult to make but once set up it isn't all that bad. You can make a reasonable mortise with a drill and chisel and tenons are pretty easy to construct in bulk on a table saw.
Wood glue is also a good idea just for the added strength, the study referenced above showed wood glue had about a 10% effect on pocket hole joints.
Based on your update:
I think the best thing here would be to drill a forstner hole 1/2" deep then drill through for a deck screw and screw the deck screw in. Then take a 1/2" redwood dowel and glue it in, then use a Japanese cut-off saw to cut it flush. You can also buy 1/2" hole plugs if you prefer not to cut them off. Match the grain direction with the plug, sand, stain, and you'll never notice they are there.
Here is a commercial tool that does what I'm talking about:
You can achieve this same effect with some redwood plugs, a 1/2" Forstner bit, and a drill.