TCHULA, Miss. — Gasoline prices reached a national average of $4 a gallon for the first time over the weekend, adding more strain to motorists across the country. But the pain is not being felt uniformly. Across broad swaths of the South, Southwest and the upper Great Plains, the combination of low incomes, high gas prices and heavy dependence on pickup trucks and vans is putting an even tighter squeeze on family budgets. Here in the Mississippi Delta, some farm workers are borrowing money from their bosses so they can fill their tanks and get to work. Some are switching...