God made the dry land appear on the third day. The dry ground is, obviously, the surface of the Earth. For our purposes the surface of the Earth is considered to be a feature of the crust.
Here's a little nuance for you: Human head don't have corners, but a beard can. We can illustrate this by putting a cheese slice on a kid's ball. The cheese has corners, the ball isn't flat.
Every time the phrase "the four corners of the earth" occurs in the Bible it's in the context of the Millennial Kingdom, after the plagues of the tribulation.
"Low lying continents with some epicontinental flooding". That's how you describe a world-wide flood without saying the words "world-wide flood."
The phrase "the four corners of the earth" occurs a few times in scripture but every time it's in the context of the millennial kingdom, it's never used in any other context.
If you want to believe that earth is flat no one's going to stop you. However nothing in the Bible, either in the meaning of words or in the context of passages, requires you to believe that earth is flat and has an edge.
God's a plan involves us going into eternity in a city which has equal X and Y dimensions, since we've satisfied the requirements of a spherical Earth, we'll model the Millennial Kingdom as foursquare.
Out of the terrible days of the tribulation, when the earth's surface will be changed in a series of earthquakes unlike anything ever seen, will come a time when all people will live in peace on the Lord's Holy mountain. Now here's a message of hope if ever there was one. This is another statement …
Some people believe that the Earth is flat. Some people also believe that the Bible describes a flat Earth. Neither of these are true. The Bible describes a spherical Earth which formed around a gravitational singularity.