Always make sure they are in phase!! The amount of times i've checked peoples systems and one or more speaker is wired up with the + attached to -
The speakers will work but the sound will be deadened due to the phases cancelling each other out....
Also, buy borrow or steal a sound meter, one that measures decibels.... You can then set the levels on the speakers correctly...
4 speaker setups.... Try to use the same type of speaker all round... They dont have to be exactly the same model but try and get from same manufacturer with same tweeter cone. This will give a smoother / balanced sound.
Subs..... Run from dedicated AMP if possible. If you can't make sure your AMP can handle 5 speakers at least and that each channel can be individually setup for Gain and Frequency blocking.
Don't mount in the Parcel Shelf!!! Most speakers sound crap and loose bass if mounted directly into a parcel shelf. If you build them into an enclosure you'll get better bass response, you can even tune the bass sound depending on the size and shape of the enclosure and if it has a port (small hole to let air out).