diff --git a/esphome/arduino8266/framework.py b/esphome/arduino8266/framework.py
index e7b55e1b7c..cded1438c4 100644
--- a/esphome/arduino8266/framework.py
+++ b/esphome/arduino8266/framework.py
@@ -68,9 +68,12 @@ def framework_package_version(ver: Version) -> str:
     package that cannot exist.
     """
     if ver.major > 3:
+        # Backend-neutral: this also fires on the PlatformIO validation path
+        # (via _format_framework_arduino_version), where switching toolchains
+        # would not help
         raise EsphomeError(
-            f"Arduino core {ver} has no known package encoding; "
-            "use 'toolchain: platformio'"
+            f"Arduino core {ver} is not supported yet; "
+            "the newest known core series is 3.x"
         )
     return f"3.{ver.major}{ver.minor:02d}{ver.patch:02d}.0"
 
diff --git a/tests/unit_tests/test_arduino8266_framework.py b/tests/unit_tests/test_arduino8266_framework.py
index 334f081bcb..c69f802c9e 100644
--- a/tests/unit_tests/test_arduino8266_framework.py
+++ b/tests/unit_tests/test_arduino8266_framework.py
@@ -25,10 +25,23 @@ def test_framework_package_version() -> None:
     # 2.6.3+ cores use the same package-major-3 encoding (PlatformIO path)
     assert framework.framework_package_version(cv.Version(2, 7, 4)) == "3.20704.0"
     # A future major bump needs its own encoding, not a doomed registry lookup
-    with pytest.raises(EsphomeError, match="no known package encoding"):
+    with pytest.raises(EsphomeError, match="not supported yet"):
         framework.framework_package_version(cv.Version(4, 0, 0))
 
 
+def test_format_framework_arduino_version_pins_all_series() -> None:
+    """The esp8266 component's PIO source formatter across every encoding
+    era, including the 4.x rejection it now shares with the installer."""
+    from esphome.components.esp8266 import _format_framework_arduino_version as fmt
+
+    assert fmt(cv.Version(2, 4, 1)) == "~1.20401.0"
+    assert fmt(cv.Version(2, 6, 2)) == "~2.20602.0"
+    assert fmt(cv.Version(2, 7, 4)) == "~3.20704.0"
+    assert fmt(cv.Version(3, 1, 2)) == "~3.30102.0"
+    with pytest.raises(EsphomeError, match="not supported yet"):
+        fmt(cv.Version(4, 0, 0))
+
+
 def test_tools_path_default_and_prefix(tmp_path: Path) -> None:
     with patch.dict(os.environ, {"ESPHOME_ARDUINO8266_PREFIX": str(tmp_path)}):
         assert framework.get_arduino8266_tools_path() == tmp_path.resolve()