and thanks for your interest in DD (and DD5!).
DD4 is a long way back in my memory, so I had to go re-read about DD4 Fighters
Broadly speaking, V4 is just not as mature as (I hope) V5 will be, and this is precisely why a V5 was required.
V4's <<will not be targeted by normal-types while there are normal targets available>> is a direct transposition of CM's <<They are the last figure in a unit that will be killed by regular missile fire of melee>>. Good pickup that it is missing from V5. I couldn't remember intentionally removing it, so I went back and checked my notes. It's in the notes, yet missing from the public PDFs. I'll have to look into that, but my initial sense is it's an unintentional omission; most likely resulting from copious amounts of copying the text around between various files.
Yes, V5's <<unaffected by fewer than four normal hits in a combat encounter>> is a transposition of CM's <<When meleed
by regular troops, and combat takes place on the non-Fantasy Combat Tables, four simultaneous kills must be scored against Heroes (or Anti-heroes) to eliminate them. Otherwise, there is no effect upon them>>.
Re applying the ignore hits rule to 3rd level fighters: Short answer is: it's your game, so you should play around and find what works for you and your players. That said, here are a couple of observations:
1) "scaling" the rule up and/or down might imply that 1st level fighter ignore 1 normal hit (which would really help low level fighters and parties go further!) and/or that a 10th level fighter ignores 10 normal hits, which might be less desirable. The printed rules are not explicit, so it's up to you how to implement it in your game.
2) While Fighting Capability of hero-1 does make a 3rd level fighter "heroic" in the binary normal/heroic sense, "Fights As" is not the same as "Is". This is evident when, for example, a hobgoblin King "fights as" an ogre, or when a 14th level fighter "fights as" a demigod. Despite their fighting prowess, the hobgoblin King is not an ogre, and the fighter doesn't actually have Zeus as a father.
Hope that helps