Very interesting modification! Could you please make a more detailed photo of the dropouts?
You had to move the rear hub axle in the dropouts by 25 mm to fit 24" wheel to 26" brake mounts...
I think 4 pistons is overkill for trials because of big weight. The only advantage is a better modulation and cooling. I really love my Avid ( = SRAM) Elixir R with 2 pistons, it has been working fine in trials since 2009!