# Stallion Express API v4

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](/v4/openapi) for every endpoint.

## Tier rate comparison (opt-in)

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](/common-workflows#compare-retail-and-enterprise-tier-rates-opt-in)
for details — the behavior is identical across v4 and v5.