Not unless you have the
iphone unlocked as I know o2 lock it to their network. o2 will do that for you (there is a form you can fill out on the o2 website) but it will take approximately 2 weeks to do.
Then you have the issue with the sim card. The iphone 4 as you know uses the micro sim which is quite a bit smaller than the standard sim card. You could always contact orange and explain you have an iphone and see if they would swap over your account to a micro sim? Not sure whether this would be an option though.
Also, if you're wanting to use the internet on the iphone you'd have to be careful. Even if you have unlimited internet on your orange contract, this is unlikely to be compatible with the iphone. The iphone tariffs come with internet and wifi so you'd have to query with orange whether anything could be done.
Best of luck!