Hospitals and doctors are charging patients with private or employer-provided coverage far more—in some cases four times more—than the government pays for the same services under Medicare.

Health-care providers say the lower payments for treating Medicare patients fail to cover their costs and are forcing them to make up the difference elsewhere. And the bill is landing on private insurers and employers who cover their employees directly.

The payment differences are a major part of the debate over the viability of a Medicare for All system that would allow people of all ages to buy into the federal program, said...