The Level 4 AI, the Half-Bloods, are harder than Level 3 Elders, but are easier than Level 5 False Gods.
The Halfbloods have the following powers:
They have 150 HP.
They have 3x Mark.
They draw 2 cards per turn if they have 2 or less cards in their hands; otherwise they draw 1 card per turn.
30% of their cards are upgraded.
When you defeat a Halfblood, you get less than 40electrum. That means an elemental mastery will get you 80 coins.
You can win some upgraded cards from the Halfbloods, however that is not likely. Be careful: if you roll 2 normal cards and 1 upgraded version of that card, you do not win any cards. For example, if you roll 2 Chrysaoras and 1 Physalia, unfortunately you do not win anything.
All Halfbloods have 64 cards in their decks(By the way, 8x8=64), composed of two halves. The two halves are determined by their names/elements. The 64 cards are divided like this : 4 quantum pillars, 20 major element pillars, 30 major element cards, 10 minor element cards.
The names of Halfbloods are based on a system of prefixes and suffixes. There is a prefix and a suffix for each element. The Mark of the Halfblood is determined by its suffix.
For example, Aethrius will be an Aether/Water deck with Mark of Water x3, and Teriel will be an Earth/Light deck with Mark of Light x3. I've fought Aquasa 2 times, its a gravity mark with water.
There are 144 combinations in total, most of them are Dual decks. However, if you encounter Halfbloods such as Lumiel or Pyrofuze, they will be Mono decks because the prefix and suffix are from the same element.
For a complete list see Level 4 Names
Of course, as it says on the God-Killer deck (Click HERE to see it) you can easily farm half-bloods. I ended up fighting lumiel twice, I won elemental mastery on one. By the way, Morofuze (Death/Fire with mark of fire) also isn't actually hard; He's underpowered because I fought him and won in 4 turns.