Code Snippets
For catalog JSON, use the REST API (beatstore/v1). For PHP, use PHP Hooks (beatstore_player_footer, beatstore_beat_offers, beatstore_resolve_preview_url, …). A plugin such as Code Snippets is still a reasonable way to load small PHP.
Removed in 4.x (do not paste these)
The following were 3.x examples. They will not run against the 4.x player.
WooCommerce single-product player — 4.x does not attach beatstore_woocommerce_single_product_player or a $beatstore_woo global. Hide a product from the player with the metabox checkbox Hide this beat from the player.
iframe postMessage cartItemAdded / cartItemRemoved — the 4.x off-site iframe does not own the cart. Play stays in the iframe. Buy opens /beatstore-buy/ in the top window so the cart cookie is first-party, then checkout.
Set-Cookie … SameSite=None for cross-domain cart in the iframe — that was for Widget 3 adding to cart inside the embed. Not the 4.x Buy flow.
If you still have one of those snippets in production, remove it after upgrading.