Will my iPhone work without a SIM card

The iPhone 5 uses a different type of sim card than the iPhone 4. If you are in the US, the sim will come with the iPhone when you buy it. Your iPhone 4 will work as a touch with or without the sim card. You will not be able to update the iPhone 4 without a sim card present.

If you don't want to use it as a phone, then yes, it will still switch on and you should be able to listen to music and do others things that don't require a sim card, such as making phone calls.

Actually, you will be able to update it just fine - I do it all the time with my old 3Gs that is used as an iPod Touch without the SIM in it.

However, if you ever need to fully restore and then re-activate it, then you will need a SIM in it (but any micro-SIM would suffice - there just needs to be a SIM in it to activate an iPhone in iTunes). As long as you never need to re-activate it, you would not need the SIM.

Also, the iPhone 5 uses a nano-SIM so will not accept your iPhone 4's micro-SIM. So you will just keep the SIM already in the iPhone 4 anyway.

In the USA even without any carrier plan associated with the device, it will still also be able to make 911 calls (Federal law requires that of all cell phones).

Well, a few days ago I found a new trick. So, what I did was I borrowed a micro SIM card from my dad, and put it in. It worked and I signed in and got my Iphone set up. My dad needed his micro SIM card back, and I gave it to him, and my Iphone was still working without a sim card. So what I'm saying is, borrow a micro Sim card from your dad or someone, and take it out less than a minute later, and it will work. I hope this helped.