From the description and SteamDB, it looks like it includes these riders, each with two bikes, two horns, and an emblem:
The description doesn't mention anything else, but the SteamDB entry lists the following locations, so it's possible they are included:
Let me know what you end up getting.
Thanks. So the Season pass activates six things - the season pass itself, the four DLCs you mentioned, and the Sliced Peak DLC.
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