diff --git a/changelog.d/5210.feature b/changelog.d/5210.feature
new file mode 100644
index 0000000000..a7476bf9b9
--- /dev/null
+++ b/changelog.d/5210.feature
@@ -0,0 +1 @@
+Add a new room version which uses a new event ID format.
diff --git a/synapse/api/room_versions.py b/synapse/api/room_versions.py
index e77abe1040..485b3d0237 100644
--- a/synapse/api/room_versions.py
+++ b/synapse/api/room_versions.py
@@ -19,13 +19,15 @@ class EventFormatVersions(object):
     """This is an internal enum for tracking the version of the event format,
     independently from the room version.
     """
-    V1 = 1   # $id:server format
-    V2 = 2   # MSC1659-style $hash format: introduced for room v3
+    V1 = 1   # $id:server event id format
+    V2 = 2   # MSC1659-style $hash event id format: introduced for room v3
+    V3 = 3   # MSC1884-style $hash format: introduced for room v4
 
 
 KNOWN_EVENT_FORMAT_VERSIONS = {
     EventFormatVersions.V1,
     EventFormatVersions.V2,
+    EventFormatVersions.V3,
 }
 
 
@@ -75,6 +77,12 @@ class RoomVersions(object):
         EventFormatVersions.V2,
         StateResolutionVersions.V2,
     )
+    EVENTID_NOSLASH_TEST = RoomVersion(
+        "eventid-noslash-test",
+        RoomDisposition.UNSTABLE,
+        EventFormatVersions.V3,
+        StateResolutionVersions.V2,
+    )
 
 
 # the version we will give rooms which are created on this server
@@ -87,5 +95,6 @@ KNOWN_ROOM_VERSIONS = {
         RoomVersions.V2,
         RoomVersions.V3,
         RoomVersions.STATE_V2_TEST,
+        RoomVersions.EVENTID_NOSLASH_TEST,
     )
 }   # type: dict[str, RoomVersion]
diff --git a/synapse/events/__init__.py b/synapse/events/__init__.py
index 12056d5be2..badeb903fc 100644
--- a/synapse/events/__init__.py
+++ b/synapse/events/__init__.py
@@ -335,13 +335,32 @@ class FrozenEventV2(EventBase):
         return self.__repr__()
 
     def __repr__(self):
-        return "<FrozenEventV2 event_id='%s', type='%s', state_key='%s'>" % (
+        return "<%s event_id='%s', type='%s', state_key='%s'>" % (
+            self.__class__.__name__,
             self.event_id,
             self.get("type", None),
             self.get("state_key", None),
         )
 
 
+class FrozenEventV3(FrozenEventV2):
+    """FrozenEventV3, which differs from FrozenEventV2 only in the event_id format"""
+    format_version = EventFormatVersions.V3  # All events of this type are V3
+
+    @property
+    def event_id(self):
+        # We have to import this here as otherwise we get an import loop which
+        # is hard to break.
+        from synapse.crypto.event_signing import compute_event_reference_hash
+
+        if self._event_id:
+            return self._event_id
+        self._event_id = "$" + encode_base64(
+            compute_event_reference_hash(self)[1], urlsafe=True
+        )
+        return self._event_id
+
+
 def room_version_to_event_format(room_version):
     """Converts a room version string to the event format
 
@@ -376,6 +395,8 @@ def event_type_from_format_version(format_version):
         return FrozenEvent
     elif format_version == EventFormatVersions.V2:
         return FrozenEventV2
+    elif format_version == EventFormatVersions.V3:
+        return FrozenEventV3
     else:
         raise Exception(
             "No event format %r" % (format_version,)