Cradle fits onto the UHI connector. I have the instruction leaflet as a pdf, can email it to you if you PM me your email address.
iPhone 4's use micro SIM cards, not nano, I've left the micro bit in my old phone (the 16Gb one) so if your phones use nano (like my new Samsung Galaxy a10) you should be able to fit it. The 8Gb one has an unactivated O2 micro/nano SIM in it.
I have Comand NTG4 which includes bluetooth, so the iPhone would connect to the car even when it was in my pocket not in the cradle, with full hands free and steering wheel control, which was handy as I keep the phone in a case to protect it (and always fit a screen protector as well). If I needed to charge it, I'd take it out of the case and clip it into the cradle. The big advantage of the cradle is that with the phone in it, it uses the car's aeriel not the phone, so reception should be loads better.
AFAIK the phones are unlocked - we're both on O2 PAYG Classic, don't know what network they were on originally, they were our daughter and SIL's phones passed down to us!
ATM Comand happily recognises wife's HTC One and my A10 (paired the A10 yesterday, result!) with full hands free etc.