2002 mercury montaineer , battery light comes on , they gages shut down couple minutes after then the truck acts like it gonna shut off....when getting a jump, the truck fires right up, couple minutes later, repeats battery light coming on, then gages go dead again and acts like it gonna shut off....battery is 4 years old

Your battery is definitely low. First clean the terminals on the battery and the inside of the terminal rings on the wires. If you have a slow charger put it on that overnight and see if it holds a charge. If it still keeps depleting or doesn't charge overnight then either your battery or your alternator are done....possibly both. So take the battery to have it load tested. Most places that sell them can test them. If the battery is done replace it then test the battery with a volt meter. If the old battery is still good put it back in the vehicle and test the voltage with a volt meter. With the engine off the voltage should read 12 volts (once its fully charged or if its new) with the engine running it should read around 14 volts. If it is way higher than that or it doesn't go up at all your alternator requires replacing. Alternators are fairly easy to replace but if you don't feel comfortable with that take it in to a mechanic.