In timeshare lingo, week 4 simply means the fourth week in a regular calender week. It is the final week in the first month of January that sometimes can overlap into February. It is a week that you can reserve, usually easily, since it is not during any particularly hot times of the year as no major holidays fall on or around this fourth week of the year. If you end up reserving this week, that means that you will have to take your vacation at your timeshare during this week in the year.
For more info, see our feature Timeshare Ownership: What Does it Really Mean?