All the API level interfaces returning or accepting null in 1.x, use undefined in 4.x.
Functions don't accept a callback anymore.
Functions that accepted an optional TransactionConfig as the last argument, now accept an optional NonPayableCallOptions. See web3-eth-contract package for more details.
List of web3-eth-ens set functions are removed in 4.x
In 1.x ens was trying to find the registry address of the current network. In 4.x the default value is set to the address of registry address in the mainnet.
In 1.x documentation it is mentioned that name,txConfig and callback are the function arguments. This is wrong, since there is one more argument, address. So the right signature is setOwner(name: string, address: string, txConfig?: TransactionConfig | undefined, callback?: ....). The same applies for 4.x, too.
getMultihash is not supported in web3-eth-ens 4.x as it's deprecated in ENS public resolver
Following functions are not supported in web3-eth-ens 4.x :
setResolversetSubnodeRecordsetApprovalForAllisApprovedForAllsetSubnodeOwnersetTTLsetOwnersetRecordsetAddresssetPubkeysetContenthash