The divide node right before the noise texture should divide by the number of pixels in the noise map, not your sprite sheet. Your sprite sheet can be whatever size you want.
And to make sure you have appropriate variation, you need to make sure your noise map uses a full range of values from 0-255, and does not bias itself towards using any values more often than others. It looks like your noise map is very white. Make sure to uncheck sRGB and use a linear grayscale texture to prevent Gamma biasing. You’ll have to change these settings in the texture panel, and then drag the new texture into the material.
On top of that, if your sprite sheet is only 2x2, that only gives you 4 tiles to randomize: you’re going to get repetition no matter what. Try using a 4x4 grid space as your minimum if you really want to get good random variation.