my opinion is forget the 4" speakers in the back - they will be useless to you.
Go with Larry's suggestion of putting speakers in the kick panels. Your absolute best way of doing it would be to build up or have built for you a custom set of fiberglass kickpanels that will hold at least a 5 1/4" or even a 6 1/2" speaker of the best audio sound that you can afford and that you like the sound of. A very good 2-way speaker or a componet set is best. Try brands such as MB Quart, JL Audio, Boston Acoustics, or other higher-end speakers for the best sound. If you also want subwoofers in the back as you mentioned, either two 8" or 10" will be plenty. The key here is twofold. First and most important is that you MUST have enough air volume in the subwoofer chamber to let the sub work properly. If there isn't enough air volume you will not get the sound out of it no matter what size sub you put in. In the rear compartment where the jack goes, two 8" subs will probably work best. Any good sub will come with specs on what the manufacturer recommends for an enclosure size. FOLLOW THOSE SPECS IF YOU THE SUB TO PERFORM CORRECTLY. Don't discount what an 8" sub can do - I've built national awarding winning car audio systems with 8" subs! The second key area on the subwoofer enclosure is that you want it to be a sealed enclosure rather than a ported box. A sealed enclosure will offer you better power handling for the sub and the sub works better and sounds better. The sealed enclosure, when built correctly, adds to the suspension of the subwoofer, controlling it's movement and damping factor. The sub can hit harder and more accurately.
The enclosure MUST be completely airtight. Just building a replacement cover panel , dropping the subs in it and laying it over the compartment will not work. Your best bet is to use the compartment as a mold than make the enclosure out of wood and fiberglass so that it fits in perfectly. The fiberglass will allow you to make it fit to the exact contours of the compartment to get as much air volume as possible plus it will not resonate from the subwoofer. once the enclosure is built, THAN measure the volume inside (filling it with water is the easiest) and once you know the volume you can determine what size and how many subs will work best whether it be one 8". one 10" , two 8" or two 10" subs. My suggestion on subs is that you look at the JL Audio brand. They are some of the best subs in the industry and also reasonable on price. They offer models that work very well in small enclosures which you will need.
For your amp, a good 4-channel amp with a built in crossover will work perfect and not take up too much room. Use the front two channels for the front speakers in the kick panels and than use the rear two channels for the sub(s). The built in crossover will than direct the proper frequencies to either the front speakers or to the subs.
It's been a while now, but I spent many years in the car audio industry previously so unlike 99% of other threads on this forum, this is one area that I do have some slight knowledge about to be able to help you.
