Since I live across the country, I can't help you with locations but I can offer this. I used to be the perennial buyer for two larger independent garden centers here and 4" pots were only available from wholesale suppliers for groundcovers or for a select variety of perennials offered very early in the season. Once past about the middle of April, these were no longer available. The reasons? A 4" pot is a very small size for many perennials and they become rootbound rapidly so the plants are only offered in these sizes as early season starter plants. A 4" pot also dries out rapidly and is difficult to maintain well once warmer weather arrives. And most customers want something larger and with more presence immediately and are not prepared to wait for small 4" plants to grow to size. Even the box stores tend to offer perennials in at least quart sizes but most often in gallons.

I worked at a local garden center for one summer many years ago, and I know what gardengal48 is saying. In addition to the upkeep (it is hot work keeping plants watered), homeowners in these parts (Monmouth) want instant gratification, and that means larger plants.

If you like herbs, Well Sweep Herb Farm in Port Murray may have some plants in 4" containers. There are many good mail order places, such as Bluestone.

I did try Well Sweep and hit pay dirt! Excellent selection of perennials in smaller pots and a pleasure to visit. Thanks.