Victor,
I am familiar with the ABS system on my car. This is what I know of it.
There are 4 wheel sensors, 3 reservoir systems.
Two for the front wheels and one for both rear wheels.
Each reservoir system has a motor that will remove execess fluid
(this you know) and also has a solinoid type clutch that will lock
that motor. This motor lock prevents the reservoir from filling by
the normal braking pressure.

If your system is working now with one wheel, then you don't need
to ask if your voltage/current is right... obviously it is.
To get 4 interrupts, just pick a processor with 4 interrupts *OR*
connect all wheel inputs to an XOR gate, then take the output
of the XOR gate and connect it to the input of another XOR gate (2)
and the input of a delay line. Output of the delay line into the
other input of the XOR gate (2). What the 2nd XOR gate & delay line
are for is to give an interrupt on any change of state (hi to low or low to high)

Also connect the wheel inputs to various I/O pins, and in your interrupt
routine, detect in software which input caused the changed of state int.

OR... use some kind of interrupt controller.... 8259???

BTW, have you thought about driving conditions???
Heat, Moisture, Cold, etc.... Automotive Electronics is not for amateurs.
Especially when there is life safety involved (ie, Braking system of vehicle)