The weather proof aspect of the label is most important when making labels for a product like shampoo. The 4x2 size gives enough room for all the label requirements and is flexible enough to work across 4 bottle sizes. This is a huge help as I don't need to completely redesign a label for each bottle size.
It is important with weatherproof inkjet label material to print at 300dpi. At the usual setting of 600dpi there can be too much ink on the label and it can run when wet, which is not nice or professional looking. At 300dpi I haven't had that happen, and the label design looks just as good.