What is the difference between `public`, `private`, `internal`, and `external` visibility modifiers?

• public: Accessible from anywhere (internally, externally, and by derived contracts)
• private: Only accessible within the same contract
• internal: Accessible within the same contract and derived contracts
• external: Only accessible from outside the contract (not internally)

contract VisibilityExample {
    uint256 public publicVar;     // Auto-generates getter
    uint256 private privateVar;   // Only this contract
    uint256 internal internalVar; // This contract + derived
    
    function externalFunc() external {} // Only external calls
    function publicFunc() public {}     // Internal + external calls
    function internalFunc() internal {} // This contract + derived
    function privateFunc() private {}   // Only this contract
}