How cut 4" wide, 2" deep hole in concrete floor
Just cut a series of parallel slices about a quarter inch apart. This should take barely a couple minutes with a diamond wheel. Since you're patching anyway, overcut somewhat so that you have full-depth cuts throughout the area needed for the laser device. 

Now take a cold chisel and hammer and just snap them off. Start by driving the chisel downward to break the first plate off, then at an angle near the bottom of each plate. 

If your grinder doesn't give you 2" of cut depth, do the work in two stages. I can't imagine the whole process taking more than five minutes.