About my purple plum tree.
Since you've only had it for 4 weeks, it's likely an establishment problem (also, you don't mention leaf spots or other symptoms.)
One possibility is that it may be drowning (too much rain and/or waterin). This is especially common in a planting hole in clay soil, where the surrounding soil does not drain quickly and the water sits in the planting hole for a long time. If you added a lot of organic matter to the planting hole, that increases the ability to receive water, but if the surrounding soil is clayish, water can't drain as fast as it comes in. This is known as the "bath tub effect."
So, if you've been watering heavily, scale back.
Otherwise, just give it time to acclimate to its new site. We do not have enough facts about the tree to give you more specific recommendations. Take a look at this publication and you may be able to pinpoint another problem: https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs...