Too much pressure iin my combi boiler????

I recently put my heating back on today and the pressure gage shot up to 4 in seconds. I then bled all the radiators in my house and it brought it back down to 3. I then turned the heating back on and again it shot up to 4. The water is pouring out of my overflow pipe and I have now had to turn the water off from the mains. any ideas of how I can depressurize the actual boiler