Everytime I turn the tv on there's alot of adverts for the iPhone 4 and something about the screen being good?

Uh-huh

They are on often but i thought the new samsung phone now is crowned with being the thinnest smartphone?

The screen is very good IMHO. You really miss it once you switch to an older (or lower resolution) phone.

Theres also adverts for dog food....

The iPhone 4 has a very good screen.

... yep.