We do memory group all together. Or it doesn't get done :). I am doing an ongoing cycle and memory work changes every 6 weeks and the old work goes into review. All of my kids should hit all of it eventually or hear it enough to memorize it. :). The longest memory subject rotation is 6 years, some are 2, others 3.
So, the 4-year old learns what we are all learning, basically. :)