Nyquist Stability Criteria
The Nyquist criterion requires to plot the response of the LOOP GAIN (feedback loop open!) and to apply the stability check. This check tells you if the CLOSED loop will be stable or not.

Question 1: You have shown the step response - is it the response of the closed loop or of the open loop? 

Question 2: What is the sign at the summing node for closing the loop ? 
Please note that the "critical point" for checking the stability is "-1" only for the simple product G*H (that means: not including the minus sign for negative feedback). Otherwise you have to investigate the point "+1".

My result: If the loop is closed (negative feedback) the system is stable. Your Nyquist plot starts in the vicinity of "+4" and circles the point "+1" twice counterclockwise. 

(It starts at "+4" because the critical pint in your plot is not at "-1" but at "+1". Hence, a shift to the right.)