What is the second expansion for Forza Horizon 4 called?

The second expansion for Forza Horizon 4 is Lego Speed Champions.

Forza Horizon 4 received two major downloadable expansions. Fortune Island came first in December 2018, adding a rugged new island, treasure hunts, extreme weather, and the chance to earn substantial credits. The second was Lego Speed Champions, announced at E3 2019 and released on 13 June 2019.

The Lego expansion transforms its setting into a playful brick-built world. It includes Lego versions of real cars, a map assembled from Lego-style pieces, and a separate campaign with its own progression. It also introduced Radio Awesome, a station built around the song “Everything Is Awesome.”

Blizzard Mountain belongs to Forza Horizon 3, while Hot Wheels was that game's later expansion. Rally Adventure is associated with Forza Horizon 5, so none of those alternatives is the second expansion for Horizon 4.