Get a 7404 Hex Inverter (6 NOT gates in one IC), 2 dual h-bridge chips and wire up two circuits similar to Fizikus’ Ar-Du
Using the above circuit as a template, you will be using 8 pins for 4 motors with full forward/reverse speed control. Adding 2 servos will only require 2 more pins, and, a range finder (rather no descript, is it IR or Ultrasonic?) will likely add only 1 or 2 more pins. You will have a number of pins left available for all manner of sensing and control.
If you notice in the above graphic, there are two inputs that carry a PWM at the end of their label.I would suggest you actually use the hardware PWM pins to connect to. You will use 4; one for each motor to control speed.
You can buy 2 motor controllers for your 4 motors or you can buy 4 motor shields. The servos and sensors are driven right off the Arduino and you’ll have space on your Arduino for it. Diagram, well, I gotta research that myself (will post it as soon as I can).