Pembina is available from several US nurseries (such as St. Lawrence), but they will likely be out of stock this late in the season. Best time to order is Nov-Jan. Patterson Pride has not yet made its way down through customs. There are lots of other cold-hardy hybrid plum varieties around though.
For Zone 4B you don’t need Mustang. It has longevity and compatibility issues anyway. Prunus Americana and Prunus Nigra are good rootstocks to Zone 3, as is Prunus Mandshurica. Krymst 1(semi-dwarfing) and Myrobalan are good for Zone 4. Some people here have used nanking and sandcherry, but they are too dwarfing to yeild much fruit. All of these should work with plums, as well as many other stone fruit.