Weeds and grass are great! You're really lucky. Maybe since you only have 4 hens, and they have a nice big run, the grass will remain. I would imagine, with time, that will change.
I have a nice big hen house, with an attached chicken run that is probably 8' x 8'. But I live on six wooded acres. My hens are locked in during the day (I have 14 hens and a roo), and then in the evenings, I let them free range outside of their run. The grass in the run died a long time ago. Now I have sand in there, and I rake out the droppings and such each day. The little flock is my hobby, so I don't mind cleaning up after them. It's my fun.
If you like your chickens, and you don't mind caring for them, it's really up to you if you want more. I don't like to keep more that 15 because then it becomes more work and less hobby.
Take care and enjoy those hens!