Hello Abdullah,
That is a great question. I always recommend purchasing the labs if you are going to be managing FortiGates if the cost is feasible for you. The lab guide and open environment will walk you through practical hands on practice as you walk through the courses. This information will be helpful with practicing hands on and not just seeing the GUI in the study guide.
For completing the NSE 4 cert, you can definitely accomplish this with just studying and knowing the material in the study guide. The questions at the end of each section are easy in my opinion, but use those questions as a guide to really understand everything about the configuration they ask about in the question. Pause at each section and make sure you understand the theory behind it. If you do this, you should have no problem passing the test.
You can also use admin guides to help understand things in more detail if needed at docs.fortinet.com.
I hope this helps and best of luck. With proper preparation, I find the courses to be very helpful and tests possible with correct study habits. Cramming for this test will almost always end up with a failure.
Cheers!
James
