Hi,
Here’s a [https://www.youtube.com/watch?v=pt2X1SoGdtE|video] that shows how to replace the LCD display for the camera.
Here’s the ifixit [guide|50289] guide which may also help as the two cameras are nearly the same
Can’t find a supplier for a HX400V LCD screen but there are plenty available online for a [https://www.ebay.com/itm/NEW-LCD-Display-Screen-for-SONY-Cyber-shot-DSC-HX400-DSC-HX60-V-Repair-Part/261855972072|HX400 LCD display - example only]. Perhaps the LCD display might be the same or you could try getting one for a HX300
Just search for ''Sony Cyber-shot DSC-HX400V lcd screen (or HX300 LCD screen)'' to find suppliers that suit you.
Hopefully thisis of some help.