For sure you can use your iPhone4 - if you upgrade to iOS4.3 - as a hot spot for your iPad. However, non-3G iPads don't have GPS capability, because the GPS functionality is on the same chip-set in the iPad as the 3G because they share the same RF (radio frequency) circuitry.
If you have a 3G iPad, though, you don't need to use the 3G capability or buy a 3G data plan to use the GPS.
As far as I understand it (I'm from the UK so you'll get better advice from US Forum members) Verizon uses CDMA technology for 3G while AT&T use GSM. If you plan to take your iPad overseas to non-US territory, you need to be aware that most of the rest of the world uses GSM, so a CDMA iPad would not be useable overseas.
Whatever you decide, you won't regret getting an iPad.
Oh - and I nearly forgot! - welcome to the Forum.
Tim
Scotland