Here's a link to the table that Mac uses to translate GPA (which is what your CA is) into letter grades and percentages:
http://registrar.mcmaster.ca /CALEN...07/sec_143.htm
It is true that you need at least a 5.0
sessional average to apply for residence in the following year. Your sessional average is just the average of your marks in one academic year. If you just finished first year, then your sessional average will be the same as your cumulative average. But, at the end of your 2nd year, then your sessional average will just be the average of all the courses you took that year, while your cumulative average will include both your 1st and 2nd year marks.
What lorend is alluding to (needing a high average to get into residence) is true in one particular case - there is always a waiting list after the lottery to get into residence after first year. You are placed in the lottery if your sessional average is above 5.0, so if you are chosen in the lottery, you're in regardless of your marks. But, even if you're placed on the waiting list, you will automatically get in to residence if you've made the Dean's Honour List that year (which is a sessional average of 9.5). They won't know this until after marks are released in May, so you'll be waiting a while to find out, but in this sense, high marks do ensure a place in residence after first year. The complete policy can be found here:
http://housing.mcmaster.ca/apply_returning_2007. htm
As for the "4.0 to pass", that varies by program. To see the minimum requirements for entering and continuing in a program beyond first year (sorted by faculty), see here:
http://registrar.mcmaster.ca /CALEN...07/sec_120.htm
Most honours programs require a CA of 6.0 to continue in them, but it is 4.0 for engineering. On the university level, the policy is here:
http://registrar.mcmaster.ca /CALEN...07/sec_118.htm To save you the clicking, it says:
All students must maintain a CA of at least 3.5 at each review to continue at the University. Under certain circumstances, as described below, students may be allowed to continue on academic probation for one reviewing period with a CA of 3.0 to 3.4. If your CA is less than 3.0, you may not continue at the University.
Keep in mind, though, that those are the bare minimums for the university - at that point you'll have dropped down to a non-honours (3 year) program, which aren't all that popular.
Hope that helped! Most of this kind of info can be found in the undergraduate calendar which can be found at the Registrar's website here:
http://registrar.mcmaster.ca /CALENDAR/year2007/