Congrats! Seq404 means you were 404th passenger to check in for this flight, if it's a 2-class A380 that's only about 65% through the total load though. However when you check in has been pretty much debunked as a reason for why one is upgraded, it tends to be based on status primarily, if you had none either there weren't any other status pax ahead of you or the outstation manager took discretion based on other factors, e.g. you had a good Y seat that someone wanted (e.g. a couple wishing to sit together) + seemed approrpiate to be seated in J class to the check-in staff (a questionable reason).