The notation of the leap day has been changed from xxxx.0.13 to xxxx.0.4. This is because the leap day rule is based on the number 4. In fact, it can be thought of as an extension of the 4x13 pattern of weeks.
Each year consists of 13 weeks of each color summing to 52 weeks (and 1 day). Each 4 year* leap day cycle (hereafter referred to as a grand year) consists of 13 months of each color (if you count the white thirteenth month as its year's color) summing to 52 months (and 5 days).
Because of the number 4's importance in the grand year cycle, it makes better sense to call this day a "day 4" rather than a "day 13".