So until LTE the MicroCell should work? But isn't the "new" iPhone suppose to be LTE on AT&T's network? So confused.
Yes, it is terribly confusing. I blame T-mobile for starting this mess, calling their 3G network "4G" to make themselves look good. And I blame AT&T for following suit. Then Apple caved to AT&T's demand to label the network "4G" when they released the 5.1 update. It's actually just an advanced form of 3G (HSPA+). Nothing out there right now, not even LTE yet, qualifies as true 4G by the definition. If you want a long and complicated read about the specifics, check the
Wikipedia article on 4G.
Basically, the new iPad supports both HSPA+ and LTE. So, since your iPad is displaying "4G", it's still on HSPA+ right now (the same network as your phone). But it's ready for when LTE comes to your area. If you're in an area where you have to use a MicroCell to get signal, though, you might not want to hold your breath for LTE coverage.
The 4S is only an HSPA+ phone. The next iPhone may or may not have LTE, there's no way to know for certain until Apple reveals it (or somebody leaves it in a bar).