Quote:
Originally Posted by
MikeM 
The effect of 4 inches of rain will vary according to the acidity of the rainfall and the chemistry of the pond. In a pond that is 5 feet deep, a 4-inch rain is adding only a small percentage of new water. In areas with acidic rain, soft pond water and low KH levels, it could cause a greater drop in pH than is desired. In other areas, the effect may be undetectable.
The KH levels you mention should be able to withstand this rain without a dramatic shift, unless you are receiving unusually acidic rainfall. Be attentive to the behavior of you koi. If they act unhappy, check the water parameters and be prepared to do water changes to restore the pond to its normal range.