i'd like to start a project to build a 4WD rover.
I want that it has to go across a a cultivated land transmitting video.
Obviously it has to be (at least) waterproof to not be damaged by the rain or i thought it could be equipped with a cover which activates through a rain sensor .
My advice would be to start with something smaller, and work your way up. Build a vehicle that can send back video remotely and pilot it around your house or apartment, before trying to build something that can work outside.
The main reason is cost; to build a machine that can travel outdoors on varied terrain for long periods (more than 30 minutes), be weatherproof, etc - will not be inexpensive. Expect to spend at least or more than $200.00 USD (for anything that isn't a "toy"). You will likely find that your machine will need to be fairly large, to cope with the terrain encountered outdoors; greater size means more weight, larger motors, larger motor controllers, larger batteries, larger budget.
If you insist on going this route though, and have the budget - I would advise you to spend the money on the chassis - specifically a Traxxas E-Maxx RTR. Everything you need will be included (including a radio); interfacing to the Arduino is simple (steering servo and ESC - just use the Servo library). Add on your video system (900 MHz or 2.4 GHz analog - or maybe an 802.11x IP camera) and your Arduino system, plus sensors, etc (you'll also want to add an SBEC or something to power the Arduino and such from the vehicle's battery).
Since the E-Maxx is a 4WD off-road and very capable RC truck chassis platform, meant to be driven in dirt, grass, damp areas (not through water, though - not without some major mods that cost $$$ - but a little rain won't hurt it) - it is perfect, and isn't too large.
For control, route the car's receiver into the Arduino, and use pulseIn() to read the signals, decode them, then send the outputs to the servo(s) and ESC. With sensors, you could use this to record your actions, or take over your actions (set it up, perhaps, to prevent the vehicle from running into a rock if an ultrasonic sensor detects something - or have a tilt sensor measure angle of the body, and prevent you from going down a steep hill that would cause a tumble, etc). Check out the code at SparkFun for the Dagu Wild Thumper control board (ATMega328 based) which is basically this entire system (the Dagu Wild Thumper is a 6WD platform that would be perfect for what you want to do as well - but it makes the Traxxas E-Maxx look cheap).