# 4 (Sequence reset)

The **`4` (Sequence Reset)** typically refers to a sequence number reset in a FIX session.

This can happen for various reasons, such as recovering from a connectivity issue or ensuring synchronization between the two parties in the FIX communication.

| Tag       | Name            | Type        | Required                                    | Description                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |
| --------- | --------------- | ----------- | ------------------------------------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| **123**   | `GapFillFlag`   | `Boolean`   | **no**                                      | Indicates that the **`4` (Sequence Reset)** message is replacing administrative or application messages which will not be re-sent. <br/><br/>Supported values:<br/><ul><li>`Y` (Gap Fill message, `MsgSeqNum` field valid). <br/><br/>**Gap Fill mode** - the message is used in response to a [**`2` (Resend Request)**](/docs/guides/exchange-fix-api/trading-fix-api/supported-fix-messages/fix-session-level-messages/message-2-resend-request) when one or more messages must be skipped.</li><li>`N` (Sequence Reset, ignore `MsgSeqNum`). <br/><br/>**Reset mode** - specifying an arbitrarily higher new sequence number to be expected by the receiver of the **`4` (Sequence Reset)** message, and is used to re-establish a FIX session after an unrecoverable application failure.</li></ul> |
| **36**    | `NewSeqNo`      | `SeqNum`    | **yes**                                     | New sequence number.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |

For a sample message and other details, see [`FIX 4.4 : Sequence Reset <4> message` in the FIX 4.4 documentation](https://www.onixs.biz/fix-dictionary/4.4/msgType_4_4.html).