tests: Test with Falcon 4.0 (#3684)

Author: sentrivana

tox.ini

@@ -431,8 +431,7 @@ deps =
     falcon-v1: falcon~=1.0
     falcon-v2: falcon~=2.0
     falcon-v3: falcon~=3.0
-    # TODO: update to 4.0 stable when out
-    falcon-v4: falcon==4.0.0rc1
+    falcon-v4: falcon~=4.0
     falcon-latest: falcon
 
     # FastAPI