Answer: It is difficult to answer without knowing what kind of shrub it is. Some shrubs will regrow thicker and faster than before at the cut. Some others will not regrow at all and may be shocked to death.
In many cases shrubs can be cut to the ground in early spring and will regrow from the base. This would probably be the best approach since you will then be maintaining them at four feet. The shrubs grow back relatively fast due to the established root system.
If they are deciduous shrubs, this would probably work. If they are evergreens, the process is riskier, but since they have already been cut back to bare stems the damage will have already been done so it is worth a try.
Again, it does depend on what kind of bushes they are.
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