I tried to build this example https://github.com/brandonh-msft/botbuilder-functions with the stable version of Microsoft.Bot.Builder v4, but I encoutered problems creating ConversationStore, all .NET samples for v4 requires a ASP.NET Core web application which obviously does not apply for Azure Functions.
And exist this nuget https://www.nuget.org/packages/Microsoft.Bot.Builder.Core.Extensions/4.0.1-preview, but is in preview, and i don't know if will be deprecate or update