I will start with a worded description, which I think will explain more than you expect. Real speakers vary resistance with frequency, but if we assume identical loads, it doesn't change the basics. Let's start with a single resistance of 4 ohms, which could be a speaker, shaker, or plain resistor.

A given voltage (from, say, an amp that is not being overloaded or over-driven) will push a given current through a given resistance. This is the simple explanation of Ohm's Law: 1 volt will push 1 ampere through 1 ohm. If you either increase the voltage or decrease the resistance, the current increases, and vice versa.

Now, if we parallel two 4-ohm loads, the results would be 2 ohms; in series, the same two 4-ohm loads would present an 8-ohm load. If we take two separate parallel 4-ohm loads (2 ohms each pair), and connect the two pairs in series, we end up with a total of (2 ohms + 2 ohms) 4 ohms, the original single value. This is called parallel-series.

Even though we have 4 ohms again, the difference is that we have 4 times the driving force and the ability to handle (or need to provide, depending on your point of view) 4 times the power. If you have a single mono amplifier, this is the way to wire it; if stereo, use two groups of 4 loads.

Now, if you connect two 4-ohm loads in parallel, and then connect the two pairs in series, you also end up with 4-ohms; this is called series-parallel. The difference between this and the first grouping is the missing one wire connecting the mid-points between each pair; the advantage is that effects of slight differences in the drivers are minimized.

Specifically with your equipment: with 8 loads, no matter how you combine them, you will end up with either a total of 2 ohms or 8 ohms, because you'll have two sets of the above parallel-series (or series-parallel) groups. Just like two individual 4-ohm loads, you can either parallel or series the two groups of four.

If you had nine drivers, you could combine them in three series-connected sets of three in parallel, and end up with a 4-ohm total. Barring that, if you must have exactly a 4-ohm load, you will have to go to four shakers, but you should have some leeway here; lets look at the amplifier.

What does it specify for rated loads? Most amplifiers will specify a certain power into a certain impedance (basically AC resistance), such as 200w into 8 ohms @ X% distortion. Your's is rated @ 250 watt, but into what load? Does it have a 4-ohm rating, or even a 2-ohm rating? If so, just connect 4 shakers in parallel (1 ohm total), then connect a second group of four (again, 1 ohm), and then the two groups in series, which would be a total of 2 ohms.

If 2 ohms, is too low, how about about another option: three 4-ohm loads in parallel is 1.3 ohms, so two of those groups in series would equal 2.6 ohms. Still too low? That's the best we can do below 4 ohms, but you can do this: make three sets of two in parallel, then connect the three pairs in series, which will total 6 ohms (2 + 2 + 2).

Whew! I'm all typed out. Let us knowwhat the amplifier's specs are, and if we can be of any more help. I'll make drawings if needed, but you should be able to draw out what I described if you do it as you re-read what I said above. Good luck.