4 hours is what a raider would do at Max level. Before max level a raider would most likey do more since they want to raid and get to that point.
A study focusing on 34,000 WoW players over the course of a year found average playtime to be 2 hours a day over that year for 75% of players and 4.8 for the remainder 25%. The study also found that a player might not log in every day (duh), but days that they DO log in, will typically have a play session longer than an hour but shorter than 5.
The last bit of information is actually really important. As it pairs well with launch hype. Studies in the past have shown players are more likely to play a game more consecutively for the first 30 days after it launches.
The first batch of 60s from casuals will not be 6 months out. Most likely it will be 3 to 4. Which would fit nicely if blizzard wants to do a 2 year classic cycle. Each phase would then be 4 months long.