You need to migrate your view to an Eclipse 4 view. That means it should be a POJO and not extend ViewPart. Since you seem to have a mixed mode you need to register it in your plugin.xml as e4view.

I'm not sure about the question.

First, in E4 you do almost everything via injection. For the EPartService it is typically fine to do a field injection.

Second, if you need to programmatically perform actions on the application model, you need to use the eclipse 4 services. Since you want to show different views dependent on your selection, IMHO using the EPartService is the correct way. If you only need to change the content of a view, you could give the ESelectionService a try.

Multiple instances of one part is done via part descriptors.

[...]
So it is important that:
* EPartService is retrieved via field/method injection
[...]

Sorry to correct you but this is NOT correct! There are multiple
EPartService-Instances and even worse they can change overtime if you
reparent your UI to another top-level window!

Typically you use the ContextInjectionFactory. And you should be able to get the IEclipseContext injected.

MPart part = partService.createPart(viewId);
IEclipseContext ctx = EclipseContextFactory.create();
ctx.set(A.class, new AImpl());
part.setContext(ctx);        
partService.showPart(part, PartState.VISIBLE);

Well, modelService.getContainingContext(somePart) is returning null because you just created the model element. You haven't added it to the application model yourself. Since you haven't posted WHERE you are trying to create your MPart, I can't tell you a better way.

Dependent on the use case and the location, I create a new IEclipseContext using the current IEclipseContext#createChild(). And then I put my dependencies in there if I need a local instance that should not be accessible in other contexts. If the created instance needs to be accessible in other locations too, you need to use the IEclipseContext in the hierarchy that fits your needs the best.

I am creating my MPart inside a class which I instantiate via ContextInjectionFactory#make. It gets the partService injected.