This is correct, staking 1 EMFOUR token gives you 4 mining power, staking 1 EM token gives you 1 miningpower. You can check for the last custom_json here: https://steemd.com/@engpool what the current mining power of all accounts is.
Currently it is 3163.84326
So the change to win at least 1.1 ENG with 1 mining power staked is around 3% every hour.