The pixels you are referring to are device independent pixels. As you can see in the documentation here:
<appwidget-provider xmlns:android="http://schemas.android.com/apk/res/android"
android:minWidth="294dp"
android:minHeight="72dp"
android:updatePeriodMillis="86400000"
android:initialLayout="@layout/example_appwidget"
android:configure="com.example.android.ExampleAppWidgetConfigure" >
</appwidget-provider>
Note how they are using dp units, not px. This allows the widget to scale correctly to different screen resolutions.
As for the screen being 4x4 cells, this is a property of the default Android homescreen manager. Regardless of screen resolution the home screen will be 4x4 cells when using the default Android home screen.
There are other home screen managers out there that have more than 4x4 cells, such as the custom home screen manager on the Archos tablets. If you want your appwidget to work in some of these other homescreen managers that do special things like increase the number of cells on the screen or allow text input and scrolling in app widgets, then you are probably going to have to write a custom version of your widget for those home screen managers.
So the answer to your question, assuming you are always targeting the default home screen manager, is that the screen is always 4x4 cells and as long as you stick with device independent pixels you have nothing extra you need to do to make your appwidget scale and work correctly on higher resolutions.