The latest stable is UnrealIRCd 5. More information on current and future UnrealIRCd 4 releases can be found at https://www.unrealircd.org/docs/FAQ#What_is_the_current_version_and_when_will_next_version_be_released.3F
There is also oldstable, UnrealIRCd 4, which will be supported until December 31, 2020. This gives users 12 months to upgrade from 4.x to 5.x.
IMPORTANT: The old UnrealIRCd 3.2.x series are no longer supported. The End Of Life announcement was in December 2015 and as of January 1 2017 all support for 3.2.x has been stopped, including security fixes. See https://www.unrealircd.org/docs/UnrealIRCd_3.2.x_deprecated