14.4 iOS crashing all my apps, even Apple Store

Before updating to the latest iOS 14.4 as told on your post what was your earlier iOS version. Then you can look into the iOS changes that has happened between the earlier version on your iPhone and this new version that you have updated now. 

This will give you a broader idea of the issue, if it is directly linked to the iOS version or is triggered by something else.

Because iOS 14.4 is only for following reasons, you may probably be carrying a legacy issue from previous iOS versions.

iOS 14.4 includes the following improvements for your iPhone:

• Smaller QR codes can be recognized by Camera
• Option to classify Bluetooth device type in Settings for correct identification of headphones for audio notifications
• Notifications for when the camera on your iPhone is unable to be verified as a new, genuine Apple camera in iPhone 12, iPhone 12 mini, iPhone 12 Pro and iPhone 12 Pro Max

This release also fixes the following issues:

• Image artifacts could appear in HDR photos taken with iPhone 12 Pro
• Fitness widget may not display updated Activity data
• Typing may be delayed and word suggestions may not appear in the keyboard
• The keyboard may not come up in the correct language in Messages
• Audio stories from the News app in CarPlay may not resume after being paused for spoken directions or Siri
• Enabling Switch Control in Accessibility may prevent phone calls from being answered from the Lock Screen

For information on the security content of Apple software updates, please visit this website: https://support.apple.com/kb/HT201222