Precise meaning of "fourfold"

Stick by your guns. Merriam-Webster’s “four times as great or as many,” like Oxford’s “four times as great or as numerous,” is designed to cover applications to both count and non-count (or mass) nouns. If you have 30 cartridges and I bring you 90 more, you have fourfold what you had before: four times as many. If you have 2.3 liters of fuel in your tank and I pour in 6.9 more, you have fourfold what you had before: four times as much. Cartridges are discrete units, and I cannot give you 2.315 of them; but the fuel is poured in a continuous stream that can be stopped at any point.

In any case, you could well argue that 3 or 3.9 is simply not “as many as” 4.

You're right, on both the interpretation of the word and the interpretation of the definition. You may go pat yourself on the back now...

However, I would generally avoid using fourfold in any situation where you really want to emphasize numerical precision. Just like double, the fact that you're using a word instead of a number seems to imply approximation.