Will this work or am I going to toast something

[Tuna141]

Yup, it's safe. All the amp sees is the 4ohm total load made by plugging in the 8ohm speaker and the 8ohm 4x12 cab (two 8ohm loads in parallel are equivalent to one 4ohm load). You could have a cab with 10 speakers and as long as it was wired so that the load was 8ohms total, you'd be fine. Just make sure that the speaker and the cab are each, indeed, 8ohms, and not less, and you'll be perfectly safe.

 

[SonicProvocateur]

Yep, two 8ohm loads each into the 4ohm taps of the Mark, it was designed that way.

In the new version of the Mark V manual there is a great reference guide to speaker impedance and what is ok to match up loading and what is not:

http://www.mesaboogie.com/manuals/MkV_100303.pdf

Ref, PAGE 53-58

 

[Monsta-Tone]

Those ratings are simply the maximum output that the cabinets can handle, not what the cabinet can put out with your amp hooked to it.
You won't be anywhere near that with your amp.

It should sound massive!!!

 

[gts]

You could also just plug the 4x12 into the 4ohm jack and unplug the speaker in the combo.
It's Totally ok to do this and the amp can take it.
In fact it's suggested in some of the C+ literature as a way to "tame"/smooth out the 60/100 models (when in 100 watt mode).
I run my 60/100 C+ this way all the time. Might be a little harder on the power tubes but the amp can handle it no problem.
Wail away and let us know how you like it!

 

[Civgeek]

You can always plug into a higher Ohm. (i.e. 4 Ohm out put jack into 8 Ohm speaker) Just not the other way. That will fry the amp because it is working too hard. Remember flow is flow so if you are trying to fill an 8" pipe using a 1" hose that 1" hose has to work super hard. Where it is easy for an 8" hose to fill a 1" pipe.