Long and technical...sorry
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Synopsis if you don't care to read:
Running a single 4 ohm gives a very small increase in SPL that may or may not be noticeable. Gaining an extra 1 to 2dB at the expense of losing the ability to run two 8 ohm cabs should be considered carefully.
When drivers come in 4 ohms and 8 ohms, often the 4 ohm driver will have a slightly lower Sensitivy rating. This may be somewhat concealed because sensitivity may be speced as
dB at 2.89V rather than
dB at 1W. The misdirection here is 2.89V at 4 ohms is 2W and 2.89V at 8 ohms is 1W. To convert a 4 ohm voltage based sensitivity rating to 1W, subtract 3dB. No change is required for the 2.89V at 8 ohm rating...1W=1W
For example: consider a 4 ohm drier that is rated 99dB at 2.89V and an 8 ohm driver that is rated 97dB at 2.89V. To really compare you need to convert to a 1W sensitivity rating. The 4 ohm cab goes to 96dB 1W/1m and 8 ohm cab stays at 97dB 1W/1m.
Next you consider the amount of power that is available. The 4 ohm cab make 96dB with 1W and the 8 ohm cab makes 97dB with 1W.
Count how many time the amp can double the power and multiply by 3. This gives us the decibel change from 1W to however much power the amp makes. Or however much power the cab can take.
For 200W at 4 ohms we get.
1,2,4,8,16,32,64,128,256 = 7 doubling of power and a little more (3x7=21dB)
For 100W at 8 ohms we get:
1,2,4,8,16,32,64,128 = 6 doublings and a little more (3x6=18dB)
If you want to also take into account the "little more," estimate and add a number between 1 and 3dB, based on how close the amp's power is to the next doubling. 200W is more than half way to the next doubling of power (256W), so I suggest adding 2dB. (21+2=23dB) This is the approximate decibel change you get when the power goes from 1W to 200W
100W is also more than half way to the next doubling of power (128W). (18+2=20). This is the approximate decibel change you get when the power goes from 1W to 100W
Finally, add the decibel change to the sensitivity ratings:
For 200W with a 96dB 1W/1m sensitivity rating. (96+23=119dB). This is the max SPL the single 4 ohm cab makes with 200W
For 100W with a 97dB 1W/1m sensitivity rating. (97+20=117dB). This is the max SPL the single 8 ohm cab makes with 100W
The difference will be even less with many amps, because they do not fully double power (200%) when you go from 8 to 4 ohms. Often the difference is more like 160%. So with an amp that makes 200W at 4 ohms, the 8 ohm power would something like 128W, instead of 100W.
1,2,4,8,16,32,64,128,256 (7x3=21dB change)
Add to the 97dB sensitivity rating (97+21=118dB) This is the max SPL 128W makes with the 8 ohm cab. Only 1dB less than you get with 200W at 4 ohms.
Keep in mind we normally say you need a 3dB change to really hear it. So gaining an extra 1 to 2dB at the expense of losing the ability to run an extra cab should be considered carefully...you really don't typically gain much. Also, pushing the 4 ohm driver harder will produce more wear and tear on both the amp and driver. This most important if you are pushing them to their limits.
When you add a matching cab you get something called mutual coupling. Mutual coupling bumps the SPL at lower frequencies by +6dB over what one of the two cabs is producing. So let's see what happens with an amp that makes 200W at 4 ohms and 100W at 8 ohms.
With one 8 ohm cab it will receive 100W. We already determined the max SPL will be 117dB
With two 8 ohm cabs, the 200W at 4 ohms will be split evenly between the matching cabs. So, each cab will receive 100W, and each cab will have a max SPL of 117dB. Both cabs together will produce mutual coupling which gives them +6dB SPL. 117+6=123dB
Summary:
--one 4 ohm cab at 200W = 119dB and,
--two 8 ohm cabs at 200W = 123db (a 4dB difference).
Also
--one 8 ohm cab at 100W = 117dB and,
--one 8 ohm cab at 128W= 118dB
Finally (see the following few paragraphs)
--two 8 ohm cabs at 100W = 120dB
Although not completely relevant to this thread, one final fact you need to know: When the amp makes the same power regardless impedance, adding a matching cab gives you +3dB from mutual coupling.
With a tube amp, the power is typically the same as long as the amp sees the expected impedance. For example, an amp might make 100W at both 4 and 8 ohms. Since the power is split between the two cabs, each cab gets 50W. The decibel difference between 100W and 50W is 3dB. So each cab receiving 50W makes 3dB less than a single cab that gets the entire 100W. But then you add +6dB for mutual coupling (-3+6=+3dB)
Let's assume 8 ohms cabs that make
117dB at 100W and 114dB at 50W. Mutual couple from two cabs making 114dB. (114+6=
120dB). So we see that when you double cabs, but hold power constant mutual coupling gives you +3dB. Essentially the sensitivity rating of two cabs is 3dB higher than one cab. To get the entire +6dB the amp must double the total output power when the second cab is added.
Mutual coupling only works for frequencies where driver spacing is within approximately 1/4 wavelength. This is why two cabs will sound more bass heavy than one.
Also when you lower the impedance driven by the amp, it has an impact on the damping factor. Definition of Damping Factor: ratio of the impedance of the speaker load to the output impedance of the amp. In some instances, the resultant change in damping can have an impact on the sound/feel.
Correction to post #37: 2.89V should be 2.83V. 
