It looks like the diagram is telling you that there is a net force of 8 N + 4 N = 12 N pointed to the right. By Newton's second law, the magnitude of the net force F is equal to the mass m times the acceleration a :
F = m a
12 N = (4 kg) a
a = (12 N) / (4 kg)
a = 3 m/s²
