Texture mapping

i am working on my terrain engine and i implemented texture splatting using 2 textures (1 base texture and another texture). now i want to blend more textures, (but my graphics card supports only 2 textures per pass). so i modified my 2nd texture to hold 4 different textures (cut and paste 4 textures like snow, sand, rock etc). now, i want to map to these textures based upon my height map info. now i have not been able to find a good way to map these textures.. suppose in my height map, the cells (10,11) has to have rock and (11,11) has to have snow, how do i map the textures or how do i arrange the 4 textures in the 2nd texture so that my texture mapping doesnt get messed up? is it possible or i have to go for multiple pass? (NOTE: my terrain is continously scrolling and new random ht maps are generated as the terrain scrolls by, so i need a quick efficient way, multiple passing might slow things down). thanks for any suggestions