How you cut them back and clean them tends to be specific to the grass, cool vs warm season, etc, but here's some general info.

3 to 4 years is about the time when some center die-out is probably happening. If these are miscanthus, they probably have a donut like shape to them by now, with dead debris in the center. For these, I would recommend cutting back hard and taking a stiff metal rake through them to clear out all debris. If they are truly donut shaped, its time to dig and divide.

I am wondering about burning too. A friend just told me that her neighbor burns her ornamental grasses in the late winter/early spring instead of cutting them down. I wondered about trying it, I have mostly miscanthus (giant) and Karl Forester.

Burning grasses is so easy! I have never seen it harm the grass, but have seen it damage near by trees and shrubs. It does create quite a fire so do be careful. I would not do it next to a house or other structure & recommend having a hose or bucket of water near by. I have not burned the giant miscanthus and am sure this would create a hellish type fire. Karls are so small it's almost as easy cutting them down, plus they are already green at this time of year.