V4 is the long-standing Stallion Express API. It uses a flat API key for authentication and remains fully supported alongside v5.
See the API Reference for every endpoint.
When the account setting Show Retail & Enterprise Rates via API (show_tier_rates_api) is enabled, each rate in a get-rates response may include a tier_pricing object with retail and enterprise sub-objects. The key is omitted for any rate that cannot be re-margined (no captured postage cost), so treat it as optional per rate. Each tier uses the same field shape as the primary rate. See the v5 tier comparison workflow for details — the behavior is identical across v4 and v5.