Unfortunately the templаte selector functionality of the series works series-wise and not point-wise and it is not possible to reevaluate the template of a single point. If this performance hit is very large, perhaps you can implement your own data-template-selector logic by using only the PointTemplate property of the series. In the template you can define your own visual that chooses its template based on the data-point (data-item) at hand. In that visual you can also implement logic that observes if the template should be changed.
Note that the approach described above may not be a very good idea if you have zooming on. This is because the chart has a virtualization mechanism where the content presenters are reused. This may result in you custom visuals changing their templates way too often, thus resulting in a more prominent performance loss.
Another option is for you to use different series for the different templates. You mention having 4 templates. Perhaps you can use 4 different series and when the template of the point needs to be changed, simply move the item in the desired collection.
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