How to cut a part of pedestal of an 3d model in Blender?

[AcademiaBA]

Hello

I have a 3D model done in photoscan, a bust in a pedestal, I need to retouch it cutting part of its pedestal. I thought it's possible in Blender, but as I'm new with Blender, after some hours still I'm not able to do it. What I need is to snap four vertices of pedestal to 3D pane, then adding a plane mesh (Add-> Mesh-> Plane) cutting part of pedestal, or maybe it's possible doing in another way.
I'll be very thankfull if somebdoy can help me with that.

Here I attach a shotscreen of my bust.

[AcademiaBA]

Hello again.

I found an easy way to dit, with Bisect option, but still I'm not able to align the vertices with the axes to make th cut not distorted 

[James]

You can get more control of the bisect tool if you use the side tool panel. Once you place your approximate bisection line, you can then set it to be exactly horizontal by setting the plane normal vector to 0,0,1 and adjusting the Z value of the plane point to wherever you want the cut.

Also press 5 on the numeric keypad to switch to an orthographic view to see better what is happening.