I have 4 inch and 6 inch in my shop, and I use them both. 4" is great for koozies, tags, pockets, and "most" sleeve prints including long sleeves. 6" is great for "most" sleeve prints on short sleeves but not long, and on things like onesies, shorts, and a whole bunch of random promo type items.
I pretty much always try to use the widest that will fit into whatever I am printing since it maxes the print area. 4" is the standard for sleeves because it fits into all of them, long and short, kids, women's men's etc. 6" is awesome for most sleeves, but some kids and some women's won't fit well on it, so it isn't really capable of being the standard.