So, how quickly after you planted these palms did you notice that they started to look funky? You mentioned that you used a root stimulator when you planted. Basically this is a high phosphorus fertilizer. It is possible that the 4 palms that aren't doing so hot got too much fertilizer to begin with. If that is the case, the leaves will start to brown and die off. Adding more fertilizer will just exacerbate the problem. (this is of course if too much fertilizer was the problem to begin with)