The free spins earned every 4 hours of active play are your Dream Spins. You can redeem these on the carousel in the lobby. It is where the games are located. If you click on the red heart in the middle of the screen, it will open the bonus credits on the carousel and spin. This is where you can redeem you Dream Spins for bonus credits or unlock new games.
Below is an example of the main lobby during using Dream Spins
After all available Dream Spins are used a count down timer will appear in the main lobby.