I don't think that iphone 4 will make any effect on computers because computer is very important machine now a days. Each and every person is using computer for various purpose. I don't think that any work of computer can be done on iphone 4. Iphone 4 is a mobile phone where you can access your mails, send messages, mms, access the internet. But you can't do the work like programming, you can't create software, you can't download any big files on iphone and many more things.
So, iphone 4 can't replace computers.
Thanks & Regards,