Can't close split screen view on iPad pro 4th generation

You can easily manage and manipulate Split and Slide Over views.

To change a Split view to full screen, place your finger one the centre of the central divider (you’ll see a little grey drag bar) and drag the divider to the edge of the screen - then release; your split-screen view will become full-screen.

You can manipulate other screen views in a similar way. To transform a Slide Over view, to Split Screen, use the grey drag-bar at the top of the Slide Over Window; drag downwards and to towards the side of the screen. As the view changes, lift your finger.

You can also change a Slide Over to a Full Screen view. This time, using the same drag-bar, drag to the top-centre of the screen, then release.

You may find these overview Apple video guides to be helpful:

After viewing, you’ll be ready for the additional Multitasking Guide:

Use Multitasking on your iPad - Apple Support

The iPad User Guide for iPadOS is also a rich source of information. Here are links to both the downloadable and online versions of the Guide:

‎iPad User Guide on Apple Books

iPad User Guide - Apple Support

I hope this explanation and additional information is helpful in resolving any difficulties you may be having with Multitasking features of your iPad.

Perhaps you can describe precisely what happens when you attempt to manipulate the Split view. Without a detailed description of your difficulties, if you are correctly following the advice already provided, it is almost impossible to provide any explicit advice as to your issue.

Perhaps this article will help you to describe the problem with sufficient detail:

Writing an effective Apple Support Commun… - Apple Community