One of the best new features of Windows 8.1 is the ability to split your Modern UI screen between up to four apps, and with today's release of the update, you can now open and snap apps side by side for efficient multitasking. The function is still limited by your screen resolution -- you get a window for each 500px horizontally on your machine. This means that if your device's native resolution is 1920 x 1080p, you can only have 3 side-by-side apps.
But even then, default display settings can prevent you from using more than two windows concurrently. If your screen resolution is at least 1500 pixels-wide, here's how to make sure you are using the right settings to open up to four apps at once.
1. Type "Display" anywhere on your live tile home page. This brings up a search for "Display".
2. Tap "Display Settings" from the results on the right.
3. Hit the dropdown arrow under More Options.
4. Select Smaller if you see that option. If your only other option is Larger, your device is already set to its widest resolution.
You should see your content decrease in size, showing more tiles on the Start screen at once. Now you're ready to snap as many apps side by side as your device will allow.