The coarse/fine India stone is good for rough work. I've picked them up at Dollar stores for a buck. They do fine for axes, sickles, machetes and the like or as the first step on a very dull knife. The soft Arkansas is much finer than even the fine India. It would be used on a moderately dull blade. The hard Arkansas would be used as a finish sharpener on a blade that is already sharp.
It's good to have all four around but those who care about blades and keep them fairly sharp can get by with the Arkansas combination stone,