4WD rover with direct drive

What are the AP configuration parameters for a 4WD (direct drive) rover?
I am using 4x brushed motors and 4x ESC connecting to the power module, one of them powering the servo rail.

The rover is similar to https://www.pololu.com/blog/92/carambola-rover

Thank you

You do not need 4 ESCs for four brushed motors. One ESC for each side should be enough for a skid steering rover. If you plan to use mecanum wheels in the future, you will need 4 ESCs.
No matter how many ESCs you use, you must configure a matching number of servo outputs as “throttle left” and “throttle right” and connect the ESC signal cables accordingly.

And the parameter settings for skid steering are
SERVO1_FUNCTION = 73 (Throttle Left)
SERVO3_FUNCTION = 74 (Throttle Right)
as given here http://ardupilot.org/rover/docs/rover-motor-and-servo-configuration.html in the Skid Steering section.
Connect your left side ESC(s) to PWM output 1 and the right to PWM output 3.

I guess the ESC needs to have a reverse function. The ones I have are pretty simple, with forward and brake.

Obviously need reverse. Are you sure they can’t be configured for reverse? Most can be enabled in a few modes with jumpers or programming.

No they don’t have reverse. Can I use two H-bridge modules instead?

One of the Sabertooth dual controllers are popular. I use brushed ESC’s for RC trucks. Many of them are waterproof.