You will be OK running the 8 ohm load on the 4 ohm head.
Technically speaking the load should match the head for maximum efficiency. I doubt that you will notice a difference.
If the head is a tube out put (specs tend to indicate it is solid state) you have nothing to worry about running a higher load on the head.
The problems arise mainly with the solid state (transistor) outputs WHEN you run a load LOWER than the output is rated for.
The load (while reactive in nature is resistance to the current flow) determines how much current will be allowed to flow from the output stage. An 8 ohm out put into an 8 ohm load is matched and the correct amount of current to develop the rated wattage will flow, allowing the speakers to get the maximum that are designed for while not exceeding the output's ability to handle the power/heat generated.
If the load is higher (as yours is) the power will be slightly less that the maximum 100w that the amp is rated to deliver. On a solid state amp this is OK to do.
If the load is LOWER than the rating of the amp..... your amp is 4 ohms..... you would NOT want to run a 2 ohm load on the amp. The lower load will cause the amp to try to deliver more than it's rated power since the resistance is lower... it looks like a slightly shorted output.... over heating will occur and damage to the amp can be the result. This would normally occur at high outputs, when you're cranking it up.
The good news is..... YES... you can run the 8ohm cab on the amp without any problems..... The better news is, if you want to add a second 8 ohm cabinet to the amp, you can.... that will give you a 4 ohm load with 2 cabs each rated at 8ohms attached.
I would not rewire the speakers..... run the 8 ohm cabinet with out fear.
As far as sound quality, you won't notice a difference. It will be loud, it will have the same tone characteristics, you will be satisfied.
If you think one cab sounds good.... get another just like it when you can afford to do so.
post edited by Guitarhacker - 2011/02/12 09:51:03