What would happen

If i were to connect an 8 ohm speaker in my 4 ohm marshall mg 100 1 x 12?

the universe would implode.

watch your eyebrows...

It's always best to make sure that the impedence matches between the amp and speaker. This is very crutial with tube amps, but usually isn't much of a problem with solid state amps, such as the MG line. The main thing you need to be sure of, is that the speaker impedence isn't less than the amps. So, an 8ohm speaker should work fine if your amp runs at 4ohms.

Use the search function of the forum to do a search on this topic. It has been discussed numerous times.

Thanks alot, I have read other topics that are almost the same as my topic, but they all have been with 4 x 12 cabs and I just really needed a specific answer for my problem. Thanks alot and ill probably stick around and keep posting on this forum regularly.