My brother, mother & I all have sibling puppies weighing 2.6, 3.5 & 4.2. We had a really hard time finding not only small enough ones, but ones that were comfortable, did not have snap lock closures as they often got caught in their fur, and no small straps as they were chewing them & making them loose. Even the fussiest one, who we had a horrible time trying to get the step-in ones on, loves hers!
We found these at a craft show - you can buy them online on their website, and they are adjustable with large, sturdy velcro (the two oldest both are wearing a Small, their brother wears an XS), can be made in any print, have a nice large d-ring, and you can get matching leashes also. My avatar pic is of Ellie in one of hers, I have a pic in my album of her walking with the leash.
J & R Painting and Crafts - Pet Stuff