I think it's too late to discuss on this but N=4 can lead to a deadlock if we consider the scenario below:
Consider there exists P1,P2,P3,P4 and R1,R2...,R6.
Now, consider that P1 and P2 completely consumes R1,R2,R3,R4 (since every process has two requests).
Now consider that only single requests each from P3 and P4 acquires R5 and R6.
Now single requests remain from both P3 and P4 (to R5 and R6). You see, now if a request from P4 tries to acquire resource of P3(R5) and similarly P3's request to P4's acquired resource (R6). This leads to a deadlock.
So this scenario COULD lead to a deadlock.
Let me know if my analysis is right.