Sandragu, PLEASE READ THIS...
Personally, I wouldn't even WORRY about adding more power to your speakers with frequency response like you have! That's more like mid-range speakers. I never buy or install ANY rear speakers with frequency response higher than about 25Hz to 35Hz in lows. I use subs, but still like a little bit of lows from my rear speakers. I didn't even know they made rear speakers with F/R that high in the low side! If you concentrate LESS on WATTS and MORE on SENSITIVITY and FREQUENCY RESPONSE, you will find you don't NEED more power to make the speakers come to life! Let me explain a bit more...
I'm 62 years old, and have been messing with car stereos since I was like 17.
Anyway, one thing I've learned through research, experience, and my MECP Study Guide is that watts are NOT the most important spec to look for in a speaker, especially if you have horrible sensitivity ratings and even worse frequency response specs on your speakers. TRUST ME HERE, you started your post by saying you're a newbie to ohms and watts, so please believe what I'm telling you since I've installed car audio in every car I've ever owned since the 70's as well as for many friends and family members. I do a LOT of research on whatever it is I'm looking for, and what I find helps me make my final decision on my equipment. ESPECIALLY when it comes to speakers! So, when I search for a new set of speakers for ANY car, I start out by comparing frequency response. I look for speakers that get the lowest lows and highest highs for their size (like 6.5's or 6x9's). Then I look at sensitivity. I make a list of the better specs I find. Sure, I also look at RMS and peak watts, but only after narrowing down my choices based on sensitivity and frequency response! You can have a speaker that claims to take a whopping 250 watts RMS and 800 watts peak, but if their sensitivity is like 80dB or 85dB, you're going to have to really crank your stereo before you start to hear the speakers, and thereby introducing distortion into them. (I'm still wondering what speakers YOU have that handle 250 watts RMS!)
As for the parallel thing, if you run your 4 ohm speakers in parallel, you turn it into a 2 ohm load, which essentially doubles the output of the amp to those speakers. You do NOT get half and half to each speaker. If your amp puts out 75 watts RMS per channel and you connect 2 speakers in parallel, the amp will now put out 150 watts RMS. Both speakers will get this power, not split between them. But you’ll need to lower your gain control if you run your speakers in parallel. IF you decide to not upgrade your speakers and still want more power, my suggestion would be to run FOUR speakers with the exact same impedance in series/parallel. That means you’re running all your speakers in parallel AND series at the same time. You will end up with the same 4 ohms that you started with, so your amp is safe and your speakers will smile.
I noticed you didn’t mention the brand of amp you have OR the speakers, and THAT will make a difference. And what size are your rear speakers? Apparently not 6x9’s. My final suggestion is to look for some replacement speakers with frequency response in the neighborhood of 30-20,000Hz or better and sensitivity around 89dB or higher, THEN you will find you don’t need more power back there!
One last note about speakers hooked to amps: most people are misled into thinking they need speakers that handle mega watts but that they should keep their amp's power output way below their speaker's rating to save their speakers, when in fact it's just the opposite. You want your amp to put out MORE power than your speakers handle, mainly so you don't have to turn things up so high to get the speakers to sound good. If you have to turn up your stereo, which pumps all kinds of watts to speakers that aren't even CLOSE to sounding good, you're wasting power and probably introducing distortion. My front speakers only handle 60 wpc RMS and my rear speakers handle 120 wpc RMS, yet my amp puts out a true 75 wpc RMS at 4 ohms yet I'm running all four in parallel with tweeters so now my amp is pumping out 150 wpc RMS yet my speakers are rated at much less! BUT, everything sounds awesome because I know how to correctly lower my gains, plus I'm running everything in the High Pass mode so they don't distort from bass. But yes, I'm running an amp that puts out way more than I should be putting into my speakers. Now if it was the other way around, where my speakers handled 150 wpc RMS but my amp only put out 60 wpc RMS, I'd have to turn everything way up before my speakers started working at their full potential but would probably introduce distortion. Distortion is what kills speakers, not power.