I'm writing a piece in the time signature of 12/8, and I want a part of it to be in 4/4. I know I can just change the time signature, but I thought of something: If you can divide a beat into three (with triplets) in 4/4, why can't you do the opposite in 12/8? Is there actually a way to do this?
Yes it is possible. Similar to triplets, there are duplets which tell you 2 notes go where 3 use to. Similarly to triplets you would group the eigth notes in two and put a two over there beams like below:
Quite often, 12/8 stuff is written in 4/4, but with a little advice note at the start indicating a triplet feel. You could write in 4/4 but say which is in triplet and which is a straight feel.