I love the 2008 acura tl, and I don't see any difference between that and the 05 which I used to own. Is the 04 any different? My main concern is the features inside the cabin, like the navigation, bluetooth, and all the other features.
how different is the navigation?

Its the same on the inside. The only minor differences are the wheels and lights. Other than that, your interior is the same.

The regular '08 TL is essentially the same as the '04/'05 TL with a few, mostly cosmetic, changes such as different tail lights, turn signal lights in the side view mirrors, different design alloy wheels, updated DVD Navigation, and iPod/Aux jack to name a few. It has the same 258 hp 3.2 V6 as before. However, there's a new top of the line model: the TL Type-S which has a bigger, more powerful engine (3.5 liter 286 hp) with either an automatic or 6-speed manual (there's no 6-speed manual available with the standard TL anymore). The Type S has different wheels, suspension, and some interior differences from the standard TL such as red gauges, Type S steering wheel, metal pedals, and navigation comes standard.

FYI: Acura is going to release a totally new and redesigned 2009 TL later this year so you may want to wait and see what it's going to offer if you're in the market for a new TL. There are rumors of Super Handling All Wheel Drive and a 3.7 liter V6 with over 300 hp.