World of Anterra Co-op and Crossplay
Steam lists Co-op, Online Co-op, Shared/Split Screen Co-op, Cross-Platform Multiplayer and Remote Play Together. The player count and the host model are the two parts still resting on secondary sources.
The one-line version
World of Anterra supports four players, online or on one screen, with cross-platform play between them. Steam's category list is the first-party source: Co-op, Online Co-op, Shared/Split Screen Co-op, Cross-Platform Multiplayer and Remote Play Together are all listed, alongside Single-player.
What Steam does not say
The category list states capability, not rules. It does not say whether split screen works inside an online session, whether guests need their own copy, or whether progress is shared between characters. Nobody has published those answers yet, and this section marks them as unknown rather than guessing.
The player count itself is worth dating: four comes from the co-optimus co-op listing, not from the store page. It is well-sourced, but it is secondary.
Read next
Co-op, crossplay and split screen has the full feature table, the three open questions, and what four players implies for the grid-based combat system — the part of the design that changes most when a second person joins.