Well, i would start by putting the cage inside the larger pen with the older guineas.
Observe what happens.
Add some escape elements to the pen some tree limbs to fly up and perch on, a bale of hay in the center of the pen, a tipped over wooden chair. With these obsticles, the younger ones can avoid being picked on by the older 2 if necessary.
Set up 2 stations of food and water.
Guineas like to do alot of dodging at each other but rarely do any do real harm. Just watch them for the first couple days.
I think they will be just fine.
As far as letting the younger ones free range. That also might need watching. I have had guineas go walk about never to be seen again. To early, maybe mine had not imprinted to home yet. Another time with young guineas, they wandered out to far in the waist high grass and became disoriented or lost. I looked for them for 3 days. Never did find them. I still feel sad about that.
Good luck!