When I talk about an OGL 4e, I’m not talking about duplicating every power, class, and feat. That would be probably not legal, and pointless since Wizards of the Coast has made the 4e core rulebooks available for sale as PDFs and anyone who wants that game can get it. What I’m talking about is building a system that works in the same way. And the way to do that is to look at the core of how 4e works.
Both fans and detractors of 4th Edition talk about it being balanced. This means a lot of things. The first is that two characters of the same level, regardless of their class, will be equally effective in play. It also means that both characters will get an opportunity to be effective in a given encounter.
The way it does this is by assuming that all characters of a certain level, regardless of class, race, etc., will have similar numbers for attack, defense, and so on. Note that I’m saying “similar” but not “identical”. There’s a range of numbers and certain variations between specific stats help differentiate between classes. But all this has been very vague. Let’s go to the first assumption.
- A character of a given level will, when attacking a monster of the same level, hit on a roll of 10 or better before modifiers.
Consider the most important “fan created” bit of reverse engineering- “Monster Manual 3 on a business card”. The math used for MM3, the Monster Vault, and all subsequent material for the game makes it so that all monsters of a certain level will have a similar attack bonus, average damage, HP total, and AC, with room for variation based on the kind of monster it is.
By that time, monster generation got simple enough that over at Blog of Holding, someone bothered to write it up as a business card. It’s a useful little thing for any 4e DM wanting to do some homebrewing (or simply fix the monsters in earlier books) so go and get that.
One of the rules of monster math is that AC equals 14 + the monster’s level. This means that a level 1 monster will usually have an AC of 15. “Soldier” monsters get a bonus of +2, brutes and
artillery get a penalty of -2. The various monster blocks come up with different ways of getting to this total, combining actual armor the monster may have with Dex or Int bonuses.
So Barbara the level 1 Fighter is attacking a level 1 monster. She needs the total on her attack roll to be 15 or better. The assumption, then, is that she has a +5 bonus to attack. Where does this come from?
There is no Base Attack Bonus or THAC0 in 4e. Rather your to-hit bonus comes from your ability score modifier, half your level, and proficiency in the weapon you use. Which ability score modifier is decided by what kind of attack it is- most melee attacks are based on Strength, ranged weapon attacks on Dexterity, and for magical classes it varies.
The first method of ability score generation in the PHB is the “standard array”, where the highest score, before any racial modifiers, is a 16. The description for each class tells you what your highest ability score should be. For Fighters, it’s Strength. So assuming you don’t make the “optimal” combination of race and class- let’s say Barbara here is a gnome fighter- you have a 16, and so a +3 mod, for the score you will be using most often to attack.
(You can aim for an 18, or even a 20 starting out if you match bonuses properly, but then it’s also possible to end up with a 14 so a 16-17 is a good midpoint.)
So Barbara uses one of her attack powers. From her stat she has a +3 bonus, and +0 from her level because you always round down. So that means she needs a 12 or better! But we’re not done yet.
Weapon proficiency acts as a bonus in 4e, not a penalty (though it could be argued the difference is mostly psychological.) If you use a weapon in which you are proficient, you get a bonus. Most of the weapons listed offer a bonus of +2, with the occasional +3. So most of the time you’ll be adding +2 to your attack total. So assuming Barbara uses a battleaxe, as she is wont to do, she has a total bonus to attack of +5. Which means to hit most level 1 monsters, she needs a 10 or better.
Now if Barbara switched to a weapon with a +3 bonus, she’d only need a 9 or better, but there are usually trade offs in damage, crit range, and so on. Let’s assume she really maxes out though, and gets a 20 Strength and has a sword with a +3 proficiency bonus- now she’s hitting on a 7 or better, an increase of 15%. “CharOp” players have found ways to rack up bonuses that are even higher! There’s clearly room for variance and optimization, but what’s important is the bottom end- someone who just follows the book’s advice to put a good score in their class’s primary attribute is going to do just fine.
What about a Wizard firing a spell? They don’t get the proficiency bonus since they’re not using a weapon- but most spells don’t target AC, but one of the other defenses. Defenses are, on average, 2 lower. Hence the math is basically the same. This little quirk in the rules helps reinforce the idea that Wizards don’t rely on weapons or normal attacks, thus protecting the niche of the martial classes. (They even have their own equivalent of the +2/+3 choice, through finding superior implements to get a +1 bonus.)
Monster AC rises with level on a one-for-one basis, but PC attack bonuses rise in a number of different ways. The “half-level” rule means that you get another +1 every even level, and every four levels you get to boost an ability score- the game assumes you’ll put that in your class’s primary. (Sublesson here: the game rewards specialization!) The game as published also assumed that you would get magic items at a certain rate, but later products introduced the idea of substituting bonuses to attack, damage, AC, and other defenses by level, so that magic items could be eliminated, saved for rarer occasions, etc. Finally, after some complains that players seemed to lag slightly behind at higher levels, the game introduced Expertise bonuses which gave a bonus to damage and attack rolls. With all those accounted for (and many 4e DMs both use Inherent Bonuses and give the Expertise feat for free around 5th level or so), the players stay within that 10-or-better range.
To put it even more simply, the game assumes that PCs hit monsters around 55% of the time, but allows for some deviation from this rule in both directions. By letting players fiddle with their bonuses here and there, it gives them a part in making their characters good at what they do, and since the DM can throw monsters that are easier or harder to hit at them, the game never feels too predictable.