A 2x2 style can be done but color become very important especially if you aren't going to make a huge amount of variations in the tiles.
I started by just seeing how the image looked when converted into black and white. If these colors are too close together, the image can be hard to read.
I notice the colors were close together so I tried spreading the colors out. I also moved some of the blocks around to stick to the 2x2 grid. I also added a small amount of blue to the rock to stop it from just being on the gray scale. Finally I tried to tone down the amount of saturation on most of the colors.
The result when converted into black and white. I think this might be a bit more readable.
