Need help. Very strange resolution behavior on iPhone

Hello

I've an iPhone 4.

When it starts, it puts itself in a resolution where i can only see a quarter of the screen.

Precisely, it starts normally, I see the welcome screen as usual, in normal resolution, then it zooms to a quarter of the image and it stays that way. Everything is functional, apart of that. When i touch a border of the screen, the screen image moves, such as it would do on a desktop Mac when you have zoomed a screen and you put your mouse on a border. This is of course very unpractical and i am not sure i can even answer a call.

the problem is that I'm travelling abroad alone (in the US) with only my iPhone and my iPad (i'm currently writing on it)

I desperately need a phone

I've tried soft resetting it (pushing home and start button simultaneously) and it doesn't change anything

My guess is that while my phone was in my pocket, i simultaneously started it and touched the screen so that it interpreted it as a change of the resolution of the screen, and i need to put it back to normal.

But i don't know how.

The phone is not jailbroken

Any idea ?