In the book Iblaagiyya Alf (sourced from Asraare' Aasaar volume 2, pages 181-82), Ali Muhammad Shirazi, the Bab writes:

'And I bear witness that after the four deputies there are no deputies for him (Imam Mahdi). Surely it has been narrated in traditions that in his Ghaybat there are scholars (who will guide the people). And I bear witness that obeying them is an obligation (upon the people) and to love them is justice. And one who denies even one of them, then surely he has disbelieved and is from the losers.'
(Ref: Asraare' Aasaar, pg 181-182, by Fadil Mazandarani)

Conclusion
Here are some deductions from the above statement of the Bab:

1. Bab swears that there four deputies were the only special deputies that Imam Mahdi ever had. This immediately disproves Bab's claim of being the Bab (deputy of Imam Mahdi). This is simple maths, if there were only four deputies then obviously Bab should not have claimed to be the fifth one. What he should definitely have considered doing is claim to be the return of one of the four special deputies like he claimed for Mohammed biin Hasan - Imam Mahdi. But then you can only be the return of one person - Imam Mahdi or his special deputy, no marks for guessing whose return is more prestigious.
   
   

2. There are no more deputies after these four deputies. This closes the door on all future contenders no matter how competent or deserving. But Bab still attempts to slip his foot through a closed door.
   
   

3. There are however scholars, who are distinct from the deputies, in the longer Ghaybat. These scholars guide the people in the longer Ghaybat. However, they are not directly appointed by Imam Mahdi and therefore cannot reach the lofty status of the special deputies.
   
   

4. Obviously Bab did not consider himself competent to be included among the scholars of the longer Ghaybat. Else he would most certainly have mentioned this in his confession given that making false claims came naturally to him.