Amplifiying 4 speakers

While it can be done, I'd recommend a decent 4 inch coaxial speaker to go in the dash.  Dash speakers are usually pretty bright anyways, and I don't see the need for adding tweeters. 

If you do go with your speakers, make sure the tweeters come with their own crossovers (if they're in an add-on set, they usually do).  Also, find out for sure what the rms rating of your 4 inchers are, then use a multimeter to set the gain to that level.  If you're unsure, use the search function in the upper right hand corner, and you should find everything you need.

If you've got a 4 channel amp, are you bridging two channels to power the sub, or do you have another amp for that?  The reason I ask is because if you are using another amp you can just use one speaker per channel- two for the 4's and two for the tweets.  You must have an outboard crossover for the tweeters, though, because many amps usually don't have crossover points high enough to run just tweeters.

I'm confused as to what you're working with.  Basically, a tweeter and a midrange should be designed to be used with each other by separating them with a crossover that was designed particularly for those drivers.  If you have drivers that are intended to be used with each other, go ahead as planned. 

It is the crossover network that is attached to the mid, not the tweeter.  The signal goes to the crossover and is separated by frequency to the two drivers.  If each of the drivers is nominal 4 ohm, the overall impedance will stay at 4 ohm because of this separation of frequencies.  You don't have to worry that you are lowering the impedance.

This tweeter/midrange network will now be thought of as "one speaker", because that is what the ultimate function is.  So just use the front two channels of your amp for the pair. 

The "full-range" driver is now no longer full range...it is a mid driver.  The crossover should be separating these drivers at about 3.5kHz, but that would be taken care of in the design of the crossover.   And again, if these two drivers were not designed to be used together with the tweeter crossover you referred to, you should go another route.

And FYI, the amp has specifications based on a 4 ohm impedance load, but it will deliver the full frequency range into any impedance load.  For example, you can run an 8 ohm load and the specs will be improved.  Conversely, you can (probably) run a two ohm load and get a deteriorated, but still full range, output.