Which spark plugs should be replaced on a 2000 MAZDA B2500 L4 2.5 Liter?

ALL eight of them! The two behind the intake manifold are kind of hard to get at. You need a long, strong needle nose pliers to work the handle of the spark plug wires. After that, long 3/8" extensions with the spark plug socket are ok to get at the plug. I suggest changing the plugs with the better double platinum ones, my engine really liked the change. TIP: put a LITTLE WD-40 on the new plug threads for easier screw-in of the plug into the engine block. I also suggest changing the spark plug wires if you are going to do this work. It is really easy except for the two behind the manifold, but you gotta take them out of the way anyway to change th plugs. REMEMBER TO WORK ONE WIRE AT A TIME END-TO-END! No mistakes happen that way. Do the hard plugs behind the manifold first! The rest will be nice and easy. tom