My ipad 4 won't turn on after I installed he IOS9.

I suggest that you try this out:

Remember before you try these to turn off Find My iPhone/iPad/iPod.

Make a backup before you try these.

(Settings>>iCloud>>Find My iDevice>>Toggle Off)

•Reboot (Hold home button and sleep button down for 10 seconds)

•Restore From Backup

•Restore As New (This will delete all the content on the device).

iOS 9 Freeze

There is a bug when updating from iOS 7 to iOS 9 (Skipping iOS 8). Luckily it is easy to fix!

Several Possible Solutions for Slide to Upgrade Problem

Here is Apple’s suggested fix: If the Slide to Upgrade screen won't go away after you update to iOS 9 - Apple Support.

Here's another person's suggestion: How to backup photos from bricked iPhone (slide to upgrade) after upgrading to iOS 9.

Here are two YouTube video suggestions you can try:

How To Fix '' Slide To Upgrade '' Stuck - ios 9 - YouTube

#IOS9 Bug - Slide To Upgrade / Apple Fix It - YouTube

Step 1 - Backup your device (This will save things like photos, text messages, etc).

Step 2 - Download iOS 8.4.1 for your iPhone or iPad.

Step 3 - Reboot phone into DFU mode - hold down the sleep/wake button and home button - when it reboots let go of power button - still hold home button

At this point it will ask you to connect your phone to iTunes - will have to connect phone to the computer.

Step 4 - Hold down the Option (Mac) or Shift (Windows) and and then click restore.

Step 5 - Browse to the 8.4.1 file you downloaded and select it. This should restore to the previous IOS version. Note: This will wipe phone.

Step 6 - Restore from backup. Hope that you backed up before attempting to update.

Step 7 - Download and install iOS 9.

More information can be found at: https://support.apple.com/en-us/HT205239