The 3000/4000 series hydraulic tanks have 4 small raised areas at each corner of the battery area. With the weight of the battery concentrated on those 4 relatively small spots, it has a tendency to wear into the plastic tank and eventually cause it to leak. The C48415 tray is Ingersoll's solution to the problem. It's a protective steel plate that goes in between the battery and tank. The factory upper hold down is still used to secure the battery into place.
I've put those C48415 trays into my 4020PS and a few other tractors as a preventative measure to hopefully eliminate any of those potential wear issues. As Jancoe said, installing it is just a matter of laying it into place, marking & drilling two 1/4" holes in the left side of the tower, and bolting it into place with a couple short 1/4" bolts & nuts.
rickr1120 ... a tip for you since you've had the tank out: If you haven't done so already, bolt the front plate (the one that goes between the tank & engine) back into place before you mark out and drill the holes for the new tray. I goofed up once and didn't do that ... with that plate out, I didn't have the hydraulic tank slid all the way back into place when I marked the holes for the tray ... so the hole alignment ended up being off a little and I had to use a round file to oval the holes out a little to make it fit.