.NET CLR - ERROR - 9521 exceeds the maximum accepted message size of 4,000

My code makes a post to a webapi from a trigger in an SP that monitors my database.

This POST works fine for msgs of up to 4000 characters but above this the SP of this error:

"ERROR -> 9521 exceeds the maximum accepted message size of 4,000."

SP:

    CREATE PROCEDURE TESTE_SP_TransactionNotification
        @object_type nvarchar(20),
        @transaction_type nchar(1),
        @list_of_key_cols_tab_del nvarchar(max),
        @list_of_cols_val_tab_del nvarchar(max),
        @ErrorNumber int output,
        @ErrorDescription nvarchar(4000) output
    as EXTERNAL NAME [Xnet.Atelie.Integration.Clr].
    [Clr.StoredProcedures].[TESTE_SP_TransactionNotification]

.NET POST Code

    protected string POST([SqlFacet(MaxSize = -1)]string message, string url, string routerKey)
    {
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "POST";

        SqlContext.Pipe.Send("Send::: " + message);

        var messageBytes = Convert.ToBase64String(Encoding.UTF8.GetBytes(message));

        var send = "{\"exchange\":\"" + this.Exchange + "\", \"routerKey\":\"" + routerKey + "\", \"content\": \"" + messageBytes + "\" }";

        SqlContext.Pipe.Send(send);

        request.ContentType = "application/json";
        request.ContentLength = Encoding.UTF8.GetBytes(send).Length;

        using (var dataStream = request.GetRequestStream())
        {
            dataStream.Write(Encoding.UTF8.GetBytes(send), 0, Encoding.UTF8.GetBytes(send).Length);
            dataStream.Close();

            using (var response = (HttpWebResponse)request.GetResponse())
            {
                SqlContext.Pipe.Send(response.StatusDescription);

                var responseFromServer = response.StatusDescription;

                response.Close();

                SqlContext.Pipe.Send(responseFromServer);

                return responseFromServer;
            }
        }
    }