None are happy without regular consistent water, but a clump that isn't performing well after four years would probably benefit from dividing...Now should be fine in your zone, Spring in areas where a lot of frost heave might be expected.

(I divided, and extracted the wild violet invading, a large clump of dwarf astilbe pumila a year ago...it was gorgeous this year!)

When you move them prepare the new plot by digging out a decent-size hole 12-18" deep and wide and mixing in at least a gallon of good compost.

Make a mound of the compost-soil mix in the bottom of your hole/s and put your plant/s on - one to each mound. Fill in around them with more of the rich mix and water in to settle the plant/s in place.

Protect from hot midday sun over the summer months.

Feed annually with a half to a full gallon of compost to each plant - spread around the leaf pile of the plant to cover the root-spread zone.

Same thing happened to my astilbe. I moved it to a different spot and it was fine. It was not doing well due to the many maple roots in the area. They like water, too. If it is getting smaller and smaller it might just need moved to a better spot and not divided. IMHO