what is the proper code for this?

in div style code. I know how to use float but only 2 divides. But in 4 divides, I don't know.
Just float them all left and if necessary add a right margin of -1px so that the borders overlap nicely. Here's an SSCCE, just copy'n'paste'n'run it:
<!doctype html>
<html lang="en">
<head>
<title>SO question 2684578</title>
<style>
.box {
float: left;
width: 100px;
height: 100px;
margin-right: -1px;
border: 1px solid black;
}
</style>
</head>
<body>
<div class="box">box1</div>
<div class="box">box2</div>
<div class="box">box3</div>
<div class="box">box4</div>
</body>
</html>
Also, if you don't want your 4 divs to wrap to the next line when the window gets resized you can place your 4 divs inside a parent div and set the width of that parent div.
Here is an example based on BalusC's code above:
<!doctype html>
<html lang="en">
<head>
<title>SO question 2684578</title>
<style>
.box {
float: left;
width: 100px;
height: 100px;
margin-right: -1px;
border: 1px solid black;
}
.parent {
width: 404px;
height: 100px;
}
</style>
</head>
<body>
<div class="parent">
<div class="box">box1</div>
<div class="box">box2</div>
<div class="box">box3</div>
<div class="box">box4</div>
</div>
</body>
</html>