Confirmed Information from one of the Developer's (Canlin):

(Will update if I get any more information)

(I've parahrased the original text for clarity)

For the Difficulty changes, the enemies do get stronger in Hard and Very Hard. I can't remember the exact value, but it's either 2x or 2.5x. I'll get it confirmed later.

Lunatic does double both the Player and Enemy damage though.

Enemy HP doesn't follow a formula (i.e a flat % increase) and instead has the enemies values set manually (for each enemy in the game).

For example: Fat Zombie's HP is: 400 (Normal) / 600 (Hard) / 1800 (Very Hard)

My Own (Probably Inacurate) Testing

These tests are from NG+ and the very first enemies you encounter, therefore this is not a very accurate test, but there’s no way in hell I’m gonna play 4 different difficulties to the end game just to test the differences more accurately, so this will have to do for now.

Difficulty Changes

The difficulty changes are written compared to Normal difficulty.

Hard

• No changes found so far

Very Hard

• Enemy HP Increased: 2.5x

Lunatic

• Enemy HP Increased: 2.5x
• Enemy Damage Increased: 2x
• Player Damage Increased: 2x