How to Unlock iPhone 4 on O2? - 1

So my mum's bought a second hand iPhone 4 from a friend and it's locked to the O2 network, however, she wants to keep the same SIM and number. How can I unlock the iPhone, preferably free of charge, so that she can use the T-Mobile SIM?

What is a data roaming cap? Do I still have to pay?

Basically a few days ago I went on a holiday to New York (from England) before I got on the plane my Iphone ran out of charge so I was unable to turn off data roaming. When I get to my hotel room I put the phone on charge and go to sleep… But when I was asleep the phone had turned itself on. In the morning I get a text telling me I had reached 98% of my £42.55 data roaming price cap (£41.70) even though I didn't even use my phone during the night… I'm on contract so does this mean I've used up all my Internet or will the price get added on to my bill? I'm quite confused (I'm on orange if it helps)

Should I buy the iPhone 4S or 5c?

I'm looking to buy either the apple iPhone 4S or the apple iphone 5c.
From people who own either one of these products, are they good quality? Do they last long?
Are there any faults/disadvantages? What are the good things about them? What are the two phones differences?
The products I'm looking at are both 16GB.
The iphone 5c is £469 (brand new from apple)
One of the iPhone 4S is at £400 ( from someone on play.com who has 98% positive feedback where he states it is brand new and factory sealed) the play.com user is GamesTradingOnline.
The other iPhone 4S is at £430 (from someone on amazon who has 97% positive feedback where he states that is is brand new, sealed in box.) the amazon user is justthejob12

So which one should i buy are why?