The following one is the interface for the IdleTokenV4 contract. Note that IdleTokens are ERC20 so they will also support methods from the ERC20 standard like approve , transfer, balanceOf

interface IIdleToken {
function token() external returns (address underlying);
function govTokens(uint256) external returns (address govToken);
function userAvgPrices(address) external returns (uint256 avgPrice);
function mintIdleToken(uint256 _amount, bool _skipWholeRebalance, address _referral) external returns (uint256 mintedTokens);
function redeemIdleToken(uint256 _amount) returns (uint256 redeemedTokens);
function redeemInterestBearingTokens(uint256 _amount) external;
function rebalance() external returns (bool);
function rebalanceWithGST() external returns (bool);
function tokenPrice() external view returns (uint256 price);
function getAPRs() external view returns (address[] memory addresses, uint256[] memory aprs);
function getAvgAPR() external view returns (uint256 avgApr);
function getGovTokensAmounts(address _usr) external view returns (uint256[] memory _amounts);