Brotato

To have the Danger 4 difficulty you'll need to beat the game in Danger 3

Danger 4 is pretty much like Danger 3 except that the enemies do more damage and hordes are introduced. I recommend to upgrade HP Regeneration/dodge/armour (in practically all the characters i found it useful) and to be patient. Try get weapons and upgrades that support the character you are playing as to make them stronger.

If you found it too hard you can still go in the settings (Options>Gameplay>Accessibility) and reduce health or damage of the enemies to 25% (recommended).