A 4 hour marathon means holding 9:09 per mile, or 5:41 per kilometer, for 26.2 miles. At that pace you pass 10K in 56:53, halfway in 2:00:00 and 20 miles in 3:03:05. Four hours is a round-number goal: it is the line between a finish that starts with a 3 and one that starts with a 4, so a few seconds per mile decide which side you land on.
The 9:09 on a pace chart is rounded. The exact pace is 9:09.2 per mile, so holding exactly 9:09 still breaks four hours, but only just: 3:59:54. One second slower, 9:10 per mile, finishes in 4:00:20. A few seconds per mile faster buys a cushion for a slow water stop or a crowded first mile: 9:05 per mile finishes in 3:58:09.
The mile table lists the clock time at every marker for an even pace and for a 2% negative split. The negative-split plan runs 9:15 per mile to halfway, which it passes in 2:01:13, then 9:04 per mile to the finish. On that plan you are behind the even column for the whole first half; that is the plan working, not a sign you are slipping. Pick one column before the race and check only that one.
Some runners reach four hours with planned walk breaks, for example a short walk through each water station. Walking lowers your average, so the running in between has to be faster than goal pace. The run walk pace calculator turns a set of run and walk intervals into an average pace and a marathon finish, so you can check whether a run/walk plan still comes in under 4:00:00.
Check the times against the course's mile markers rather than your GPS watch. Courses are measured along the shortest route a runner could take, and few runners follow it exactly, so a watch usually reads a little long: running 9:09 by the watch tends to be slower than 9:09 by the markers. For other goal times, the marathon pace chart lists every finish from 2:30 to 6:30.