Right now the 3.5 week difference in age means a lot. In the weeks to come it will matter less. It's like the difference between a newborn and a toddler. Right now you wouldn't put them in the same Sunday School class, but you could have 3- and 5-year-old children together.
Keep the lil' ones in a separate brooder box for a few weeks, until they gain some strength & sturdiness, and get some growth. Gradually you can introduce them to the older chicks in the weeks to follow. Start with cages/pens side-by-side. Then put them together with close supervision, while giving them all treats. If they seem to be getting along, then you can begin to keep them together.