My mini tree is around 4 years old now. It had a mass of blossoms in the spring but they have all dried up into dead clumps. In fact this happens most years.
What am I doing wrong?
As you've described the blossoms 'all dried up into dead clumps', I'm assuming those clumps remain on the tree. If that's the case, your tree is probably suffering from Blossom Wilt, a fungal infection. Treatment involves spraying with copper oxychloride (if that's available where you are) just before flowering - next year, now of course. In the meantime, prune out affected spurs (the branchlet behind the flowers) and inspect the leaves and stems closest to the blossoms - if they have pinprick sized buff coloured raised pustules, or there's die back, cut back beyond that point.
If the clumps of blossoms fall from the tree and don't remain withered there, this advice isn't relevant.