If a lipoma causes severe back pain and is about 4" in diameter would Medicare cover it?

If a lipoma is rapidly growing in size, causing pain or discomfort, or is associated with any other symptoms then usually insurances including medicare will cover the surgery to remove it.

Yes, Medicare will cover your procedure (you will have an applicable coinsurance and/or deductible!) if your lipoma is growing or causing pain. With a 4" lipoma, it may be best to see a general surgeon as this may or may not be something that could be done in-office; it may need to be done at a surgical center. "This answer has been solicited without seeing this patient and
cannot be held as true medical advice, but only opinion. Seek in-person
treatment with a trained medical professional for appropriate care."