Chris,
You would think that ohms and it's relative measurement/calculation would be rather simple but impedance is not so simple as it would seem. The reason is that while ohms of resistance in, say a resistor, remains consistent in a circuit, the ohms of a speaker resistance (impedance) varies with frequency. In a single audio program, there are highs, lows, mids, and every frequency in between and during operation, the ohms that the amplifier will actually see varies from very low (maybe sub-2 ohms) to very high ohms.
When you add a tweeter into a circuit, you are doing so, presumably through a high-pass filter cap. Therefore, only the very high notes are passed onto the tweeter. As the most demanding and strenuous frequencies are in the low z's, and since those frequencies are filtered out and not passed onto the tweeter, therefore, the amp only really sees a tweeter when it is passing high notes. The rest of the time, it's like it's not even there. What's this all mean? Well, it means that you simply can't calculate the impedance of a speaker circuit like this and say that with the addition of this or that driver, you will get "x" impedance. In general, you can safely add a tweeter into a circuit through a filter cap and the extra load to the amplifier is negligible.
Not so if you add other "full-range" drivers that get the full signal however since it will then mean that the entire signal is being shared all the time, and not just on selected notes or hz. Now since your speakers are already 4-ohm drivers, it is inadvisable to connect another driver in parallel since that could indeed halve the imedance (if 4-ohms). If you connect a 6 or 8 ohm driver in parallel, then the impedance will be between 2 and 4 ohms but higher than 2. There is a formula to calculate but unless you want me to, I won't bother because I don't recommend it. Now you can install them in series and that would be perfectly OK for your amp, but I also do not recommend it.
Here's the reason why.... Unless you are using matched drivers, running them in series or parallel is unwise because one or the other will almost surely have different specs and characteristics. That does not mean that they will complement each other. Rather, you will find that the louder one will basically drown out the other one. If you don't believe me, put 2 boomboxes next to each other and turn one up to a certain loudness. Then turn up the other one to a significantly more powerful volume setting. I bet the only thing you are going to end up hearing is the louder boombox. The softer one will be invisible. Remember that expression, if you fell a tree in the forest, and nobody is around to hear, does it make a sound? Yeah, it's like that except that in this case, it truly is that it's making noise but you can't hear it. All right, to satisfy those who will say maybe the specs aren't that far off? Let's just say that it doesn't take much mismatch to make one driver disappear. Maybe not entirely, but enough to beg the question why bother with the extra driver if it's just going along for the ride with nothing to add?
Here is where I think you can do something that will improve the sound overall and accomplish what you desire... go ahead and add another driver, prefereably a high/midrange and not a full range driver (which will do a crap job of competing with the 10 incher). Install a 3-way crossover. When you do this and if the crossover is properly designed for the number of drivers you plan to use, then the entire impedance in the array remains unchanged regardless of how many drivers you have. And even better... the lows go to the woofer, the mids go to the midrange, and the highs go to the tweeter. Each driver performing at it's optimum environment. Basically the crossover divies up the signals based on the Hz and sends them off to the proper driver. Even though the woofer is being fed the full signal (in a system without a crossover), the highs are wasted since it isn't capable of reproducing it, or at least not capable of doing it well. The only reason you don't see an inductor in series (to filter out the highs) is because #1 it adds cost and #2, the highs don't hurt the woof like the lows will hurt a tweeter. However, it's wasted energy since it gets the same signal that the tweeter is getting.
Hope this helps you to understand it a bit better.