same math for a calculator.. 8(ohms) 1/x, plus 4 (hit 1/x key for it too) = number.. hit 1/x = answer.

the 2.67 ohms might be a little light (too close to a short) for some amplifiers, theres ways around it with resistors.
the 4+4 ohms in series with a 2 ohm resistor for a 10 ohm string parallel the 4 ohms, goes to 2.85 ohms.
same resistor added to the 4 ohm side instead, 3.43 ohms, closer to "safe"..

add 4 ohm to the 8 in series and 2 ohm to the 4 parallel.. 4 ohms total. if the pair of 4 ohm speakers in series sound too bright, try adding another 2 or 4 ohms resistance there, is one way of going after balancing the sound output, and you might find you need a lot more than that (especially if dealing with shrieking peizo tweeters omg).
more likely you wouldnt notice much if any real differences for adding some low value resistances that can help keep an amp section from cooking.

another solution (maybe preferable?) is using a 3-way speaker crossover, bass channel to the woofer, mids and highs paralleled to the speakers in the backbox.. see what you get with an ohm meter. this can be good with older machines having a single mono output for divvying out the sound. resistance or L-pad might need adding inline to the mid-high, or to both mid and high freq lines seperately, easier to tune sound how you like going that way.

I play with very small audio amps a lot, like 12v and 7-14w per channel small. a lot of people dont understand that just 10 watts of power through very efficient speakers, can be loud as all hell! power hungry subwoofers and big amplifiers to feed them are nice too sure, but a lot of times they really arent needed. some of my low budget favorite high efficiency woofers lately are from "MCM", direct from their site or through amazon. paper cone cloth surround 12 giving 97-98 Db from one watt is pretty stompin, their similar 10's and 8's (all 8 ohm) are too for their size.

long story short (HAH!) try to keep it 4 ohms or more, and you CAN squeeze big sound outta very little power.
"acceptable impedance"? personally I wouldnt risk there, but I sure do like good audio.