pressure on guage is 4 bar what can i do to solve it its a alpha HE cb 25 b?

There should be a Pressure Release Valve on your system that should have discharged the water through a safety pipe outside. This should have happened at 3 bar. If the PRV is faulty or missing you will need to either change it or add one. This is very important. If you want to know what happens if you dont go onto UTUBE and type is mythbusters exploding water heaters

tyemanone
December 2009

Bleed the radiators of water to reduce the pressure

CKING
November 2009