Nearly 4,400 small businesses in California will share in $3.5 million in rebates from insurance giant UnitedHealth Group Inc. this summer as insurers nationwide prepare to return millions of dollars to customers as a key benefit of the federal healthcare law kicks in. The first of these California rebates, amounting to about $98 each for nearly 36,000 small-business employees and dependents covered by UnitedHealth, comes because the company's spending on medical care fell short of new government requirements.