Hanging Kitchen Wall Unit

I've got a very heavy 900mm x 900mm kitchen wall unit to hang, it comes with two hangers at the back and two wall plates that are held on by two screws, but are 4 screws enough to hold such a heavy unit? If I was to provide extra support how could I do it? The top of the units are only 60mm away from the ceiling and one side of the unit is next to a cupboard, so I can only easily access one side and the bottom of the unit.

Also the wall I'm attaching it to is plaster over something very...

Hanging Kitchen Wall Unit