bi amping with 4 monoblocks ?

I agree with Edwyun. I would also add that unless an active crossover is used, there is no real benefit to using 4 amps rather than 2.

^Well, there might be some benefits.
It might allow for more available power.
Many amps run in Class A up to a point, then work in class AB there after. If one pair of amps are running the demanding woofers, it could allow the other pair to maintain Class A operation longer in the typically more critical to human hearing of midrange/treble.
It might allow the amps to run off separate power outlets, which might be advantageous.
The amps might run cooler, with smaller cabinets that might be sturdier and less prone to vibrations, and easier to move.
It might help keep cable runs tidier.
It might allow for easier upgrading as funds become available.
In the event of an amplifier failure, it might allow continued use of the system during the time that the damaged amp(s) is/are repaired or replaced.
Oh yeah, they system will be more ready, should an external active cross-over become available. :-)

It depends on the power rating of the amp versus the power handling characteristics and efficiency of each individual driver. If the amps are sufficiently powerful and the drivers relatively efficient, then there would probably be no advantage to using 4 versus 2.