Fab Fours Inc., one of Lancaster County’s small-industry success stories, is doubling its workforce, saying big sales growth is driving the expansion. The company, which manufactures aftermarket bumpers for vehicles, announced plans Friday to add 88 employees and spend $5.7 million adding equipment to its plant off Riverside Road. It now has about 80 employees. With the new workers, Fab Fours’ employment will have increased more than tenfold since it moved to Lancaster County just seven years ago.