The Merkava achieves an incremental increase in crew protection by placing the engine & transmission up front in the tank hull behind the armor of the front glacis. If a tank round were to penetrate the frontal armor, then the mass of the engine takes the brunt of the impact and theoretically the crew survives.
I think the problem is that man-portable AT weapons usually impact on the sides & rear. The armor is thinner there and of course the engine placement has no benefit.
As for “Chobham Armor”, I don’t know if the Merkava IV makes use of that technology.
The Merkava tank is unique in that the engine is in the front, not the back. This is done to create another buffer for the crew.