The only wasy to increase the lifting the lifting capacity is to increase the bores of the lift cylinders (in other words buy new cylinders) or increase the hydraulic system pressure, say from 2000 PSI to 2500 PSI.
The first option would be WAY expensive (figure a minimum of $750, but probably much more) and WAY dangerous since the rest of the loader structure is not designed for the increased payload the bigger cylinders would allow you to lift. The second option, increasing the system pressure, depends on what pressure you're running now. You could have a defective pump, pressure relief valve, etc. that's now allowing you to reach the correct pressure.
There was a "running line" change to the 4210's hydraulics at some point in the model year so that is ended up with the same hydraulics as the 4310/4410. I wonder if yours has the change or not. My 4310 w/ 430 loader & 73" bucket has no trouble picking up heaping bucket loads of anything including clay, crushed limestone, broken concrete, wet topsoil, sand, etc. My hydraulics are running at 2750 PSI, stock(for now /forums/images/graemlins/grin.gif ).
