So just to be clear, how do we know the Earth formed 4.6BYA?
The main force driving scientists to conclude Earth's age is radiometric dating. Although this method can be difficult at times thanks to plate tectonics melting rock back down to magma, we can use the Zircon rock in Australia to conclude that the Earth is at least 4.374 billion years old.
And what about those craters?
Well since Earth is in the same solar system as the planets with visible craters such as Mercury and Mars it is highly likely Earth had many craters at one point which are almost all extinct due to plate tectonics.
So the rocks on Earth's surface are much younger than Earth itself?
Exactly right Laura! Plate tectonics cause volcanoes which spew magma from convection currents onto Earth's surface which then cools into rock and because of the law of superposition we know that this new cooled magma is younger rock than the rock it cooled upon. This can be backed up further using radiometric dating.