Since you have a four foot drop, I would be tempted to dig a trench from the pond to the river and drain as much as possible. If you can get a four-wheeler close to the water's edge in dry times, then dropping that water level will get it even drier. And since you can get a four-wheeler on it, seems very likely you could get a track hoe there too.

Then I would dig a deep hole in the pond, or very near the edge, and start pumping. Use that hole to get the water to drain into it, go deeper if necessary. Then start digging out the main area, making adjustments as needed. This is based on if your pumps can keep up with the inflow. If not then you might go around the edges with a long reach hoe.

There is a dragline set-up called a sauerman's bucket that will let you dragline any size/distance. But it's a much slower process, and most can't handle the operation of it.

If you dig a trench, you may want to put a pipe and valve in it somewhere so if the river rises it don't flood your work area.