Yosemite beta testers got a new treat from Apple yesterday, but Cupertino is continuing to refine its next major release of Mavericks by seeding the fourth beta of OS X 10.9.5 to developers this morning.
OS X Mavericks 10.9.5 build 13F18 is light on new features but heavy on bug fixes. Apple asks developers to focus on Safari, USB, USB Smart Cards, Graphics, Thunderbolt and GateKeeper. Developers can grab the new beta from the Mac Dev Center.
Apple also released the sixth beta of OS X Yosemite yesterday that brought a bunch of new wallpapers to the Mac, new System Preference icons, and Do Not Disturb mode.