You can download old IrfanView versions from these sites:
Oldapps.com or Oldversion.com 

There were numerous changes, here are only the more important things.

Version 4.52 ( - CURRENT VERSION - )

(Release date: 2018-12-12)

• Improved Paint PlugIn stability
• Support for DPX and CIN formats (DPX PlugIn, thanks to Patrick A. Palmer)
• Support for FLIF format (Free Lossless Image Format, PlugIn)
• Support for HEIF format (with installed HEVC extensions/codec, see Win10)
• New option in Advanced batch dialog: Set minimal dimensions for resize
• Option to save settings in Canvas dialog
• Insert Text dialog: CTRL + click in image adds the text permanently
• Command line: /filepattern option: accepts multiple extensions ("*.jpg;*.png")
• Improved support for high DPI displays
• Improved JPG2000 format loading
• Several PlugIns are changed/updated, please install the newest versions:
  http://www.irfanview.com/plugins.htm

[ Older versions - History]