In this case "has" is certainly the preferred option.
"Contains" seems to indicate that something resides inside something else, or that something is an ingredient of something.
With electronic equipment the features the object has are being used by it, they are not simply inside (or in the case of a screen, they are actually on the outside.
Milk contains fat.
This box contains my stamp collection.
My car has leather seats.
My TV has an internet connection.
In the case of your iPad:
The new iPad has a 4.5" screen.
Alternatively, since we can think of the iPad as "showing off" the really nice screen:
The new iPad sports a 4.5" screen.
Off-topic, I'm not following these things very closely, but I have a 10" tablet. Is 4.5" very small for a tablet, or do you mean an iPhone?