ValarieJ;
You just need to spread the load over a larger surface area than the four table legs presently provide to protect your carpet and carpet pad underneath.
We say your table including Millie weighs in at 600 pounds. If each leg has a 1-inch foot, this means that there is 150 pounds of weight on top of each leg. With the vibration of of quilting, this weight over time can damage the carpet and pad.
All you have to do is spread the weight load over a larger surface area to protect the carpet. You had to right idea with the 2x4s, but I believe that is overkill.
I would suggest you go to Home Depot or Lowe's and purchase a 2-foot by 2-foot piece of 3/4 plywood. Have them cut the board down into four 1-foot by 1-foot squares. Depending on the store, they normally will give you one or two free cuts. Then cover both the top and bottom of the boards with carpet that matches your present floor. Place the Millie table on top of the squares. The 150-pound of weight under each foot is now spread out over 144 square inches, which reduces the weight to a little over a pound per square inch.
Using 1/2 inch plywood and making the pads 6-inch by 6-inch would lower the tripping hazard as you walk around the table, and still do a good job of protecting your floors. The military does the exact thing when they place plywood under the rolling stock wheels to protect the floors of the aircraft. Think how much stress stiletto heels can put on a floor.
Best of luck with your project.
Cagey