Level 4 ATK and DEF Rules
The ATK and DEF of a Level 4 or lower monster tend to be limited by two unwritten rules: the "2000 ATK Rule" and the "2300 DEF Rule". These rules state that for a Level 4 monster to have an ATK or DEF equal to or higher than these values, it must have some kind of negative effect or other restriction (such as being a Nomi monster). For example, "Meklord Emperor Wisel" is a Level 1 Monster Card that has 2500 ATK and DEF; however, it cannot be Normal Summoned or Set.
Rule of 2000 ATKEdit
The "Rule of 2000 ATK" states that if a monster has 2000 or more ATK, it must have some negative effect or other restriction to limit its playability. For example, "Thunder King Rai-Oh" has 1900 ATK and is thus permitted to have a beneficial card effect, but "Panther Warrior" and "Zombyra the Dark" have 2000 and 2100 ATK respectively, and thus have the negative effects of requiring a Tribute in order to attack and losing ATK upon attacking, respectively.
Several Level 4 Normal Monsters have 2000 ATK, outclassing monsters with 2000 ATK and negative effects. They all have either 100 or 0 DEF. There are currently 10 such monsters: "Alexandrite Dragon", "Gene-Warped Warwolf", "Leotron", "Megalosmasher X", "Mekk-Knight Avram", "Mystery Shell Dragon", "Phantom Gryphon", "Photon Token", "Shiny Black "C" Squadder", "Zombino".
Level 4 or lower Monster Cards with 2000 ATK or moreEdit
There are currently 95 Level 4 or lower monsters with 2000 ATK or more in the TCG/OCG.
Rule of 2300 DEFEdit
The "Rule of 2300 DEF" states that if a monster has 2300 or more DEF, it must have some negative effect or other restriction. For example, "Gladiator Beast Hoplomus" and "Gear Golem the Moving Fortress" have 2100 and 2200 DEF respectively, and are allowed to have positive effects, while "Big Shield Gardna" has 2600 DEF and has the negative effect of being forced to switch to its weak Attack Position after being attacked.
Currently, no cards in the OCG/TCG break this rule.
There are currently 14 Level 4 or lower monsters with 2300 DEF or more in the TCG/OCG.