DALLAS—Southwest Airlines will add a fourth fare category as part of changes designed to attract more business travelers and boost revenue. The new fare level will be priced higher than Southwest’s cheapest tickets but below the airline’s top two fare categories. Southwest executives think this will fill the large price gap between the cheapest fares, called Wanna Get Away, and more-expensive tickets. Consumers who buy the new fare level, called Wanna Get Away Plus, will get 33 percent more frequent-flyer points than the basic ticket, and they will be able to transfer the value of a ticket to another Southwest customer. It’s the first major change in Southwest’s fare structure in 15 years. Airline officials said the changes announced Thursday will take effect in May or June. Airlines frequently tinker with fares and fees to squeeze more revenue from passengers. In recent years, Delta, American, and United have reacted to …