4.0g Red Dot with the bullet at 1.135 COL should get you 900 fps out of a 4.5" barrel. I shoot 180g cast bullets out of my M&P 40 and have gone down as low as 3 grains Red Dot with full cycling. 2.8g caused the slide to not lock on the last shot about 50% of the time so 3.0g was the min for me.
The 4.0 load with the 155g bullets should give about the same recoil as a full power 9mm which means "pretty light". (IPSC Power Factor = 145). You also have lots of margin and can probably go up to 5.0g and still have some safety so long as you don't seat the bullet much shorter.
Typically what I do when "working down" loads is start at a reasonable load and start dropping until I start having cycling problems or start getting sooty cases. Hodgdon Clays is a great powder for this because it is very good at burning cleanly and generated high pressures without high velocities. But Red Dot/Promo is almost as good, it just leaves a lot of unburned flakes as you go down in pressure.
Hope that helps.