Integratinghens

Good afternoon,

I have 2 Rhode Island Red and 2 Isa Browns. They are about 12 weeks old, I think. I started off with 4 chicks because I was afraid that I would bite off more than I could chew. However, now I'm feeling that I could handle 2 or more extra in my flock. Would it be safe to purchase two more laying hens right now? Or should I wait until my current hens are ready to lay? I've read that integrating can be hard depending on the age groups.