The latest stable is UnrealIRCd 4.2.4. More information on current and future UnrealIRCd 4.x 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 UnrealIRCd 5, which is currently under development. You can post issues and requests here about that too. If you do report 5.0.0-alpha/beta issues, be sure to run the latest version.
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