The setup:
1/ Have a material that is a 2 x 2 grid of 4 images as the base color
2/ Have a blank alpha, so when placed in the level it is there but masked, so not visible
3/ The goal is to randomly show 1 of the 4 images, or all 4, or none.
The Issue:
I can make this work by having 5 different alphas and switch between them. (However many more are needed for all the possible combinations of the 4 images, 0, 4 * 1, 4 * pairs, 4 * threes, 1 * all.
The question:
Is it possible to make a alpha as needed by adding offset squares and then feeding that to the material in a Blueprint? This in turn could be a function that is fed a number between 0 and 5 to produce the alpha.
The bit I am stuck at is making the alpha in real time. eg if this is scaled to a checkerboard of 64 squares having the relevant number of alphas quickly becomes impractical.
You can generate procedural tiled effects using this sort of approach:
or you might be able to do something with a flipbook node in combination with this.