Oh boy....
Ok, you have two wiring options. Series and Parallel. Series, add the resistance..ie...4+4 = 8 ohms. Parallel (For your applicaiton) halve it..4/2 = 2 ohms.
You can run an amp as you stated, but keep in mind, usually sub amps with a x1 rating are at 1 ohm. Not always, but alot of times thats the case. You'd be good with an amp that does around 600 watts at 2 ohms. Its no worse to underpower subs (ie..using an amp with less power then what the sub can handle) as it is to overpower them (Using an amp that puts out more power then rated). However! In your case, I would recommend using an amp with a touch too much power. Alot of times, people with amps that dont push their subs hard enough for them end up cranking gains and boosts way up and run the amp into clipping. Clipping is bad. Really bad. Think of it as head room, your subs will start complaining from too much power before you clip on an amp that can supply the power wihtout clipping.
SQ is sound quality..ie...to sound good. SPL is Sound Pressure Level....ie...One goal, to get as loud as you can and damn how it sounds.
Now, the pwoer ratings on subs are alwasy to be taken with a grain of salt. Install can really screw with sub power ratings. Sealed box, just give'm 350 and be happy. Ported gets a bit funky. Above tuning frequency, you can run around the rated power. At tuning frequency, you can give'm 2, 3 even 4 or more times the rated power for short periods of time without undue concern, provided you know when they've had enough. Below tuning frequency and power handling drops FAST because the sub unloads from the box...ie...2, 3 or even 4 times LESS then rated power could still be too much, depending how far below tuning you go.
Are you doing the install, or is a audio shop?