If you only have a smaller tool, like a dremel, then cut around the whole coral about 1 inch from the top. You score a circle around the whole piece, then stick a flathead screw driver in the score, tap it with a hammer, now your 4 inch tall coral is only 1 inch tall, then cut it up as you wish...
Just about every coral I get, I first cut off the excess rock/ skeleton. It doesn't harm the actual coral at all... it has long outgrown what you cut off. That's the plan anyway,