Measure the top of your coffee table and purchase a glass table topper the same size as your table surface and as thick as possible.
Place small rubber or wooden spacers or "feet" under the glass on top of the table surface at the corners. These will hold the glass above the current table surface.
Set glass over the table and check the new height. If necessary, use taller spacers to raise the height.
Turn the table over and mark the center of each leg at the bottom.
Drill a hole straight down into each leg to a depth of at least 1 inch.
Cut a dowel of appropriate size---at least ¼ inch in diameter---into four equal pieces to use as pegs.
Put carpenter's glue on the end of one peg and insert it into the hole in the leg as far as possible. Repeat the procedure for each leg.
Measure the depth of the interior hole of one of the wooden balls or finials by inserting a small wire or stick as far as it will go and marking the depth at the surface.
Cut each dowel peg in the table legs to that length.
Test fit a wooden ball or finial onto each---pushing it down onto the peg as tightly as possible until it is flush with the bottom of the leg. Cut the pegs as necessary so that each decorative ball or finial fits perfectly flush against the bottom of each table leg.
Remove the balls or finials and put glue on the end of the pegs. Replace each on its peg and wipe away excess glue before it dries.
Finish the leg extenders with paint or stain to match the table, and let dry.
Turn the table back upright and enjoy the new height.