BGR reports that the fourth beta version of iOS 8 will be seeded to developers on Monday, July 21. The release date would mark a two-week gap following the July 7 release of the third beta. There had been roughly a three-week interval between the second and third beta seeds.
BGR is usually fairly accurate in their reports that predict future iOS beta release dates, usually landing within a day or two of the actual release date, barring last-minute delays by Apple.
We’ll keep our eyes out for the Beta 4 release of iOS 8, and let you know when it lands.