4 rockford 8's?

I will be installing four Rockford Fosgate P2D28's in my car. I don't have any amp(s) yet and I'm not sure what I need to power them. Can I use one amp to power all four subs?

I think the hardest part for that setup will be finding an amp that's 1 ohm mono stable(yet, I could be wrong).  You may be better off wiring them for 4ohm mono or getting some with 4ohm coils and wiring it for 2ohm mono.  I'm sure someone else will chime in with some other options as well.

Yes you can.  The diagram you posted indicates using one 1-ohm stable amplifier.  If getting one of those is not in your budget, you could otherwise wire the subs to 4 ohms as linked here and use a standard two channel amp, wiring to it in bridged configuration.

A third option, just in case you like a lot of options, is to use 2 mono amps instead of one.  Your selection of product is much greater than getting a 1 ohm stable amp and might be easier on your pocket.  Use the middle wiring diagram (2 ohm) here for each amplifier.

The subs don't require anything more than 1 watt.  They are rated for the amount of power they can handle continuously;  it's a continuous, or thermal, power rating (aka RMS).  No, the different ways of wiring coils / subs together doesn't change the amount of power each sub is built to withstand.  The wiring methods, instead of changing anything about the subs, serve to change the impedance of the wired group of subs so that the amplifier performs the way you intend it to. 

You have 4 subs and each is rated identically for continuous power handling.  Add those 4 numbers together.  That total should help you select an amplifier whose RMS output into a 1 ohm impedance is in the ballpark of their combined capacity.  Ignore "peak" numbers associated with either subs or amplifiers.

Thanks I'll plan that way then. I appreciate all the help.