overgrown myrtle comm

I have 2  dwarf myrtle bushes have become too large for their spaces. They are about 4 1/2' tall and 8 ft around. There are leaves only on the outer 4" or so . They are at least 40 yrs old and have been shaped round with only light trimming over the years. What is the right way to bring them down to size both height and circumference?  How far can they be cut back? If cut back beyond outer leaf growth, how long before new growth appears? I would appreciate any information on the proper way to prune these shrubs. thank you