P = perimeter = 2L + 2W = 86 cm.  Also, L = W + 4.  Subst. W + 4 for L, 
P = 2(W + 4) + 2W = 86 cm.  Then 2W + 8 + 2W = 86 cm, and 4W = 78 cm.
Finally solving for W, W = (78 cm)/4, or 19.5 cm.
If W = 19.5 cm, then L = W + 4 cm = 19.5 cm + 4 cm = 23.5 cm
The rectangle's dimensions are 19.5 cm by 23.5 cm.