[VoodooChile]

'Bloodgood' is the industry standard cultivar for hardiness in Acer palmatum. Make sure your grower is producing vigorous stock from true types.

Still,the bottom line is you can't ask the plant to achieve beyond its limits, and zone 4a is definitely asking Japanese Maple to overachieve.

Your only option would be the creation of a micro-climate; try to site the plant with winter wind shelter, not in a low space where cold air collects, protected from southern sun that will induce early bud break, perhaps adjacent to a poorly insulated house or next to a body of water that flows through winter, i.e. a spring fed stream or winterized water feature.

A better bet is to pursue some of the other Asian Maple species. There are growers in the Madison area of Wisconsin who produce decent specimens of Acer pseudosieboldianum, Full-Moon or Korean Maple, and Acer triflorum, Three-flower Maple, and Madison is solidly zone 4. Even with a hardier species of Asian maple you would again achieve better results, a longer-lived specimen less prone to die back, by siting in a micro-climate.

Of course, Full-Moon Maple and Three-flower Maple aren't the same tree, but both have their own merits, like spectacular fall-color and a fine texture, and there is something to be said for one less "purple beast" in the landscape.

In zone 4a, Japanese Maple will always be a rich man's annual.