Will apple replace my iPhone 3GS with an upgrade?

I got a hand me down iPhone 3GS from my dad and it's about 4 years old now. The silent button won't work anymore, the 3G has never worked and there's a big crack in the back of it. My mum sent away her iPhone 4S as it completely broke down and they told her she would more than likely get a new one. I was thinking of sending mine away, because my dad gets them by his work and he would be able to get them to send it away even if it's out of guarantee. Will they replace it with an iPhone 4?

No. They will replace it with a refurbished iPhone3.

Yeah probably a refurb iphone 3G you have to pay for a new one