A 4 ton boulder versus a major flood, the rock will move, nothing in a river is permanent, gold and boulders will find a new place after a major flood. The river is always fluid not static.
If you must blast a huge rock, not much explosive is required if you create a shape charge. Place a conical or cone shaped charge on top of the boulder and initiate. Place the cap at the top facing DOWN or a loop of det cord at the top.
honestly, thats horrible charge to use on a rock. Thats the whole reason you drill a hole to put the charge in. Shape charges are for breaching, not blasting.