WASHINGTON — Gasoline prices dipped to just under the $4 mark for the first time in more than five months — good news for consumers who are struggling with high prices for many other essentials. AAA said the national average for a gallon of regular was $3.99 on Thursday. Prices have dropped 15 cents in the past week and 68 cents in the last month, according to the auto club. The shopping app GasBuddy reported that the national average was already down to $3.98 on Wednesday. Falling prices for gas, airline tickets and clothes are giving consumers a bit of relief, although inflation is still close to a four-decade high.