The fourth day is the fourth day of creation week which has been counted according to God's Enoch calendar. The fourth day of the Gregorian week is called Wednesday, but Gregorian does not obey the third sentence: "days 365 and 366 are nameless", thus fixing the sabbath cycle to itself and not the vernal equinox, the "great sign". It gives (pagan) weekday names both to the 365th day and Febuary 29th/366th day on leap years. Therefore Gregorian "wednesday" usually does not coincide to the day name "fourth" on Enoch. Furthermore, enocal dates are expressed as "day x of month x", like we read in the bible in Ezekiel and other places and are not sabbatarial cycle ordinal numbers at all.
The first day is called day one and the first sabbath is day four of the first week, and every seven days from that for the other 51 weeks of the year. As stated in line three of the Enoch synopsis, days 365 (Job's birthday) and leap year day 366 (yes, on the same schedule as gregorian) have no date on the calendar. Qumran temple scroll confirms this stating the feast of weeks (shavuot) is on day 15 of month three, always on the first day of the sabbatarial, seven-day cycle. It's easy to know leap years because 1) Enoch's leap year coincides with gregorian and 2) the vernal equinox will simply show up a day later, which can also be confirmed as far ahead of time as one wants, online, or on a sundial at home.
You'll notice various authors differ on the first day of the calendar, whether it's the first or fourth day in the sabbath cycle. The latter is correct because He set our game in motion on the fourth day (Gen 1:14, see below) and rested on His seventh (not the clock's, the calendar's or ours). The key is to understand that Yah's day one "light" isn't the physical light that set this clock in motion.
And God said, "Let there be lights in the expanse of the heavens to separate the day from the night. And let them be for signs and for seasons, and for days and years...." (Gen 1:14)
"Light" in the first day refers to Yaweh Himself, which is good, which He then separates from the darkness (evil). Physical light and dark do not need separating, but good and evil certainly do! Day one of creation week is a paralell to Adam and Eve and the garden, who, by sinning/making a bad mistake, understood the difference between good and evil. God adds to His Terrarium duality now, i.e., distinguishes the teams in the game of life. BTW it is through this game that we are to learn, grow, and ultimately decided winners in heaven or losers in hell. First creation day light is not physical/calendar "signs for the years" (see above, Gen 1:14).