prøv dette, du kan bruge et alias for COUNT(*)
<?php
$connection=mysqli_connect("host", "username", "password", "database");
$sql="SELECT COUNT(*) as cnt from database_users";
$res = mysqli_query($connection, $sql);
$users = $result->fetch_assoc();
if ($users['cnt'] < 4) {
?>
// html goes here, outside of the php tags
<?php
} else {
echo "Sorry, you have reached the user account limit.";
}
?>