I think 3 inches is plenty of substrate, and 4 inches is not too much. The problem I have with slopes is that everything submerged in water weighs a lot less, so most substrates are very light weight. That means they can easily shift their position in the tank. When I make slopes, within a few months I have near level substrate again. Or, the water circulation is such that a local eddy will scoop out one area and build up the adjacent area. Or, certain fish will do their own slope making.
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