So we’re talking about four year old “plum” trees; would that be European (prune) or Asian (plums)? ...not that there’s a tremendous difference, though European's seem a bit harder and disease resistant.
At this moment a large percentage of the tree’s nutrients are being stored in the roots. Trees that young can not afford to waste stored energy. And by removing worthless wood, that energy would push into more productive wood - with a few ‘water suckers’ you could either remove or train. If you were to wait until June, or even April/May… the limbs you plan/need to remove will be loaded with that stored energy and growing. By removing them ‘then’ you’d be wasting that energy while lessening the tree’s ability to replace it, thus weakening it’s root system as well. Avoiding that is the best reason for pruning now.
To avoid disease I’d wait until we’ve a stretch of predicted dry weather – but don’t hold your breath
At the first sign of said weather (two or three days..), I’d prune. Remove unnecessary shoots to the point where they emerge. And research has shown it’s best to let a flush-cut wound dry and heal without sealing it, I agree. Don’t make too wide of cuts by cutting too close to the base limb (from where they emerge), but don’t leave ‘stubs’ either.
As most plums are beginning to send up sap with swelling buds… sooner’s better than later, in MHO
… Have fun – and welcome to our forum ~