Hi, I want to get this phone as a very special present for someone for Xmas.

I do not know much about mobiles or iPhones.. I cannot decide whether to buy unlocked from Apple store or take up a 2 y.o. plan from Swisscom.

Person whom I want to get a phone to, has already pre paid swisscom card and ideally wishes to keep this number.

However swisscom estimate delivery is 4 weeks so it is a bit too late for Xmas. also if I want to keep it as a surprise but to move pre paid number to a new phone how will it work technically?

if I get unlocked phone from Apple, will it be possible to use pre paid card for internet, emails, calls etc...?

I am really not that good about phones, connections, internet, etc.

thanks in advance for your advice.

If you get the unlocked one from Apple, you will be able to use it with any operator. If you get it Swisscom locked, you will be only able to use with Swisscom (straight away anyway, there are ways .....). I am not sure if the Swisscom contract iphones are locked though.
Either way, very likely you would have to "trim" the simcard, as the iphone 4 uses the microsim format, so straight out the Christmas package he likely won't be able to plug the simcard in.
The contacts are the easiest to be transferred via the pc with setting up the two phones to be synchronized through e.g. Outlook, or with a bit of care through the "cloud", e.g. google contacts. All depends on the actual phone that special someone has now. If his prepaid plan includes data usage (if I remember those used to be pretty expensive on prepaids, although situation may have improved since I last checked), then he'll be able to use it for calls, emails, internet and everything.
Have I missed anything?

Sounds about right...

yes they are - and the iPhone is the only phone that is sold with a 2-year contract but only comes with a one-year warranty.

If you can afford it, go for an unlocked one from Apple by all means.