NEW YORK -- U.S. gas prices jumped back to $4 a gallon on average Monday, as Washington’s latest fighting with Iran renews strain on the flow of fuel worldwide. According to motor club AAA, the national average for a gallon of regular gasoline is now just above the $4 mark. Where gas prices stand todayAgain, U.S. gas prices are now back at the $4 mark. But the latest gas prices follow an uptick in oil costs seen over recent weeks. Meanwhile, U.S. gas prices hit a four-year record of more than $4.50 a gallon in May.