You are going to want to make a current driven set-up.
As high power LEDs get hot and/or age they will change their characteristics. You are going to want to account for that and the easiest way is driving a 4.2A (or, advisably 4A with brand Unknown eBay type) and let the voltage be what it is.
The cheap Chinese drivers usually drive a current about 10% below the normal advised LED current for that type and dimension. The best driver I have seen (German built) was 97% efficient given a narrow input voltage band and exactly the specified LED. My own measurements of cheaper models come between 70% worst case and 91% best case. This differs from type to type and greatly with input voltage.
Many cheap modules also have an input pin somewhere on the main chip that allows 0-2V or PWM (3.3V or 5V) dimming, if you know what to look for. In case you want to add that later.
I have not had the need to import and/or use 100W 24V COB drivers yet, so no specifics on that exact type.
If you use an Atmel to drive it with PWM, again make sure you use a current measurement for your feedback parameter.
As for driving them with AC, there are undoubtedly modules that do exactly that, but I don't know any specifics.
If you build something yourself for 4 lamps, that are well protected from touching and moisture (this includes the LED's metal base plates and such) you could potentially regulate the current with an Arduino through 4 lamps in series with a voltage taken directly from a rectified AC.
This, however is risky and requires a lot of careful experimentation and testing with a low voltage, low current power source to avoid damage (48V, 3A and one LED PWM'ed, for example, no risk of death and if you accidentally short the PWM transistor the supply will limit to 3A, leaving your lamp in tact).
And officially you need to perform power factor correction, but if it's for hobby / one-off there's worse devices in the world that used to be mass produced, so don't worry too much.
If you are working with just the LEDs, keep a VERY close eye on temperatures as well, they need a lot of help getting rid of heat and just the LED's back surface will absolutely not be enough for more than a watt or 2.
Let us calculate the resistance of the LED at the operating voltage:
R = V/I
V= 21 v (typical), I = 350 mA, so
R = 60 Ohm.
The LED driver operates at maximum 24 volt with 700 mA, so it works with maximum 34 Ohm load. Any load with higher resistance will give an unpredictable current result that may damage the load. So you can not simply connect 35 ohm resistor with the 60 Ohm LED in series.
To divide the current, you can connect the load in parallel (not in series) with 60 Ohm resistor. If it is difficult to find exactly 60 Ohm resistor, you can use 56 Ohm/10 Watt resistor that is available in digikey. It gives 338 mA and 20.276 Volt in the LED.
Best Answer
If your input voltage is going to vary that much, you're going to have to start with a power conversion stage to generate a local stable power source. You have three choices - step down to less than 7 V (let's say 5 V), step up to more than 24 V (let's say 26 V) or a buck-boost type converter which aims for somewhere in the middle (let's say 12 V).
Given the extra complexity of the buck-boost, I would rule it out personally. You then have two different operational currents which you can calculate - 22 W / 5 V = 4.4A or 22 W / 26 V = 850 mA. Neither of these are horribly large given decent amounts of copper trace on a 2oz / sq. ft. board. However, your ohmic power losses in traces / PWM device channel will be just over 27 x higher with the low voltage version - but they may still be insignificant as a percentage of the total output power.
As you can see here, boost converters tend to be slightly less efficient than buck converters, so you're trading losses in the converter stage against losses in the traces / PWM control. Personally I would opt for the high voltage option because it places lower demands on my PWM control device, but overall efficiency is hard to call between the two, and will likely be down to the quality of design execution over the choice of path. If you have the chance, mock up both approaches on a board using a dummy load in place of the LEDs and work out which matches your needs best.