Week 4 is the Steelers bye week so there will be no start/sit advice this week. Instead, I will give you some general rules for who to play and who to sit on any given week.
Do pay attention to the defense your player is facing. Especially focus on the opposing team's average points allowed per game vs. a specific position. Use this when you are trying to decide on which one of your two borderline starters to play in a specific week. However, Don't over-react to an opposing defense and sit an elite player because he is facing a great defense. For example, this week Demarco Murray is going up against the very talented Bears defense who has given up the 3rd least fantasy points per game (FPPG) to running backs. However, Murray is a top 10 RB in the league and should not be sat because of this. However, on the other hand, Brandon Lloyd is going up against the awful Bills defense who has given up the 3rd most FPPG to wide receivers so even though he is a borderline starter, he should be started this week.
Do play backups who are replacing injured star RB's. This is a philosophy I have employed with great success in past years. Typically, when a team has a star RB, they focus their gameplan on him and just because he is injured, they won't make huge adjustments to their gameplan. Some examples of this are Andre Brown last week replacing Ahmad Bradshaw or Michael Bush replacing Matt Forte last week. However, Don't use the same philosophy with wide receivers. If a star WR is out, don't try and find his backup and plug him in as you're starter. There are simply too many talented players on an NFL team that can make plays when a star WR goes out to trust a backup.
Do pick your starting defense/special teams based on who they are playing. Every week you should be taking a new defense unless you have one of the top 3 to 5 defenses in the league. The performance of a defense is pretty much dependent on who they are playing. The Bears are an awesome example of this philosophy. In week one, they faced the pretty average Indianpolis offense who was breaking in a rookie QB and they put up an average 14 fantasy points. In week two, they were facing offensive juggernaut Green Bay and they put up a measly 9 points. In week three, they were facing the dreadful Rams offense and they put up 23 points. Some teams you should try and have your starting defense be facing are the Jaguars, Browns, Rams, Dolphins, and Bucs. These are five very bad offenses that even below average defense will be able to produce against.