I'm not really sure how the 4" springs would work with your 5.5" kit...that's an interesting bit of suspension geometry due to the 1.5" reduction in lift. I will say that mixing and matching between systems isn't forbidden by any means, but that I also cannot tell you how well it will work in this case.
In my experience, Currie makes a very good product across the board. I don't have experience with their springs, directly, but based on their other products that I've seen/used, I have no doubt that the springs are of very high quality.
Ride quality depends on a lot of things...vehicle weight and weight distribution, tires/tire pressure, shocks/shock mounting locations, etc, etc, etc...the Currie springs may change the ride quality, but I couldn't predict in what specific way.
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