Not to play devil's advocate or anything, but the standard added time for a goal celebration is 1 minute, and 30 seconds for a substitution. Man Utd scored on 95:30 or thereabouts as far as I'm aware, which is 1:30 more than the added time displayed before a goal and substitution.
Also, the board only displays rounded minutes added on, this time 4, when in fact it could have been 4:30 or 4:50 by the referees watch.
If, and this is an if that might be hard to swallow, you add all those up you end up with about as much time added on as was given, and there wasn't any conspiracy.