MTG Can Krosan Drover reduce a card with X converted mana cost X equals 0 conversion if the controler pays 6 or more mana for it also if they can would only paying 4 plus -2 mana cancel the effect?

When working out the cost for a card with X, first decide what X will be, and then apply any cost reducing/increasing effects.

If a card costs G(X) (eg, Hurricane) then if you decide X will be 6, then Krosan Drover makes it so you only need to pay G + (4). If you decide X will be 0, then you only need to pay G, but you could decide X is 2 and still pay only G. The point is you need to declare what X will be.