I have a 3D grid of 100mX100mX50m, with 4m intervals among all the points. On the other hand, I intend to copy this small 4mX4mX4m block onto the grid to form a massing. The problem is I want fixed number of blocks copied which is about half the number of all the points in the 3D grid. SO how do I create iterations of different ways of arranging this fixed number of blocks within that 3D grid, in order to have different results of massing.

I don't quite understand what your setup is or what you're asking for, but the sort SOP might help you, I think. That will let you randomly rearrange the numbers of the points you're going to copy too and then you can just delete the range you want to go away (or the range to keep)

Maybe putting up a scene file would help demonstrate your issue.