docs(docs/breaking-changes/v4.md): rewrite v4 breaking changes docs to clarify precautionary angle

Author: michaeljaltamirano

docs/breaking-changes/v4.md

@@ -1,5 +1,9 @@
-# Breaking Changes in v8
+# Breaking Changes in v4
 
--   Adds support for `bun` and `pnpm`
-    -   Updates logic for existing `npm` and `yarn` detection, and so may
-        inadvertently change behavior. Cutting as new major as a precaution.
+-   Adds support for `bun` and `pnpm`. Cutting as a new major for visibility
+    into expanded package manager / node runtime support.
+    -   These changes update logic for existing `npm` and `yarn` detection,
+        though care was taken to maintain the existing behavior, and no changes
+        are required when upgrading from v3. (The new major is also a means of
+        flagging visibility into this changing behavior, and so if new errors
+        pop up under this major, there may be an unintended regression.)