Hi there. 

I'm assuming that you are asking if you'll be eligible for writing the IIT JAM examinations if you complete your undergraduation in 4th year instead of the 3rd year. 

According to the latest updates, the eligibility criteria mentions that you need to have a Bachelor's degree and no where does it mentions anything regarding years or gaps. 

Apparently then you are completely eligible for writing the said examinations. 

For more relevant information kindly refer to the link given below:

https://www.google.co.in/amp/s/university.careers360.com/articles/jam-eligibility-criteria/amp

Wishing you luck!