To clarify what cribble has supplied:
You advance straight to 4th year pay AFTER completing one year of service AND having all ATPL subjects having been confirmed in your position (end of probation). You remain on 4th year pay until completing 4 years service and then you keep advancing up the scale as your years of service tick over.
The rates regarding incentive pay get quite a complicated explanation which I don't have the inclination nor time to reproduce here, however for arguments sake those are the minimum rates you will get paid. If you are a 737 pilot you start earning extra incentive for every hour over 55 hours. If you are on the other fleets you start earning extra incentive after 60 hours.
Is that any clearer? I am not so sure myself after all that.