Lets say you have two of those speakers. Each has two wires on it. What you wanna do is wire from the jacks "tip" to one side of the first speaker, then the other of that that same speaker to the second speaker, then from the second speaker to the "ring" of the jack.
Two four ohm speakers, in series... result is 8 ohms. If you use all four speakers, then daisy chain them all together... total of 16ohms.
Use the appropriate inputs.
There is something to watch for. Speakers are marked with a + sign, or "dot" on one terminal. That may or may not be consistent with the wire colors there - car speakers being what they are. Its important to make the marked terminal the "input" terminal. Think of it as from "tip" to input, then leaving the speaker to the next speaker "input", then back to the "ring" of the jack.
You wont hurt anything, but if you reverse one speaker, it will be out of phase and the whole thing will sound terrible.
Wattage - at first glance, it appears that a 40w speaker would work on a 5w amp, no? And two... rated now at 80w would be huge overkill, no?
NO.
In fact, those car speakers are not rated in RMS watts, but in some arcane weighted "created to fluff the numbers" rating system. They call it "music power" or call it "program power" or "peak" power, or all sorts of things, but its not RMS wattage, which is what your amp is rated in. Your little 5w head probably wont destroy them, but an amp of a maybe just a bit greater power will. The speakers probably can handle about 5w RMS each, if that.
Given all that - the TONE you get will be pretty shrill. Why? Because guitar amp speakers have decent low end but max out at about 5000 to 6500 Hz. Your car speakers sacrifice low end, and will give you extended treble for that "crisp" in program music (bells, snares, cymbals). But... have fun with 'em.
