WASHINGTON — Businesses have hired 4.5 million workers under a new program that provides tax breaks for hiring unemployed workers, the Treasury Department said Monday.  It is unclear, however, how many of those workers would have been added without the tax break.  President Barack Obama signed a law in March that exempts businesses hiring people who have been unemployed for at least 60 days from paying the 6.2 percent Social Security payroll tax through December. Employers get an additional $1,000 credit if new workers stay on the job a full year.  To find out more information, please follow this link.