A sample UIKit app that sets up a four column layout with new iOS 14 API on UISplitViewController.

FourColumns.swift

import UIKit

class FourColumnsContainerViewController: UIViewController {
    let outerSplitViewController = UISplitViewController(style: .tripleColumn)
    let innerSplitViewController = UISplitViewController(style: .doubleColumn)

    let primary = makeContentViewController("App")
    let secondary = makeContentViewController("Files")
    let mainContent = makeContentViewController("File Content")
    let inspector = makeContentViewController("Inspector")

    init() {
        outerSplitViewController.primaryEdge = .leading
        innerSplitViewController.primaryEdge = .trailing

        innerSplitViewController.preferredSplitBehavior = .tile
        outerSplitViewController.maximumPrimaryColumnWidth = 120
        innerSplitViewController.maximumPrimaryColumnWidth = 200
        outerSplitViewController.primaryBackgroundStyle = .sidebar

        innerSplitViewController.setViewController(mainContent, for: .secondary)
        innerSplitViewController.setViewController(inspector, for: .primary)
        outerSplitViewController.setViewController(primary, for: .primary)
        outerSplitViewController.setViewController(secondary, for: .supplementary)
        outerSplitViewController.setViewController(innerSplitViewController, for: .secondary)

        // MARK: - Boring stuff below here

        super.init(nibName: nil, bundle: nil)

        addChild(outerSplitViewController)
        outerSplitViewController.didMove(toParent: self)
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(outerSplitViewController.view)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        outerSplitViewController.view.frame = view.bounds
    }

    @available(*, unavailable)
    override var splitViewController: UISplitViewController? {
        nil
    }

    @available(*, unavailable) required init?(coder: NSCoder) { preconditionFailure() }
}

private let makeContentViewController: (String) -> UINavigationController = { title in
    let vc = UIViewController()
    vc.title = title
    vc.view.backgroundColor = .systemBackground
    return UINavigationController(rootViewController: vc)
}

class SceneDelegate: UIResponder, UIWindowSceneDelegate {
    var _window: UIWindow?

    func scene(_ scene: UIScene, willConnectTo session: UISceneSession, options connectionOptions: UIScene.ConnectionOptions) {
        let windowScene = scene as! UIWindowScene

        let window = UIWindow(windowScene: windowScene)
        window.rootViewController = FourColumnsContainerViewController()
        window.makeKeyAndVisible()
        _window = window
    }
}

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
}