It involves a very prominent circle that you probably already noticed at some point.
You have to get to a higher spot to solve it.
You'll be looking downward as you solve it.
Symbol #4 is drawn from the very tallest tower. But to reach this, you have to have solved a number of other Town puzzles. And those puzzles are designed so that you can't solve them until you've learned the rules from all the other puzzles on the island.
When you solve enough of the Town puzzles, you can climb to the top of the tallest tower in Town.
Climb up there, look down at the black hole, and you'll quickly see how to draw the path.
Complete spoiler:
The circle is the big black hole above the Projector room.
From the correct spot (and height), you can combine it with those black wall fragments to make a path.
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You see this while you're in the Town area.
The circle itself is just to the west of Town, but when you solve this, you'll be back in Town.
Roll over each black area below for hints.
Each new hint gives away more than the previous one.
There are actually a few high places in Town that allow you to trace the paths for three different symbols -- symbols #2, #3 and #4 on this North face of the obelisk.
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(Note that Symbol #4 requires solving some other puzzles before you can reach the correct vantage point.)
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Map
Town Obelisk
North #4
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Intro
This one's purely visual. It simply involves standing in the right place.
(Note that you probably can't solve this particular symbol until fairly late in the game.)
THE WITNESS - ENVIRONMENTAL PUZZLE HINT GUIDE