I have a USB car charger from TomTom. Can I use it to charge my iPhone 4, or will that damage my phone?
Yes, it works great. The iPhone charges over USB like many other devices. The iPhone can accept more current than the USB specification, so you might have a slower charge than a charger that provides that extra wattage, but the iPhone circuitry is designed to take any normal USB charge and properly handle the charge which includes pausing it when the battery is full.
Also, just so you know, because I was curious about this myself, it is safe to charge the iPhone/iPod with the larger 10W iPad AC wall charger. http://support.apple.com/kb/HT4327
You can charge an iPhone with most any device which has a USB port. USB is designed such that it starts with a small amount of electricity and the device requests more if it wants it – like to charge, for example. I have never heard of a device being damaged by USB power and I sincerely doubt it's possible without some kind of hardware fault to begin with.
In fact, the problem most people find (especially with the iPad) is that the USB port can't provide enough power. This is the cause of the "Not Charging" notification which you get on iPads. I suppose that as the iPhone develops in power (with rumours of the A5 iPad chip coming to the iPhone 5 in September) this may become the case more generally.
As far as iPhones not getting enough juice, it's generally fine. Especially with the 12V cigarette lighter socket to which you are probably referring. The one exception I have found is the USB ports on in car stereos which are designed for memory sticks only (in order to play MP3s loaded on the sticks). Those just aren't built to supply enough power, and won't charge an iPhone (in my experience).