It happens. For 90 years the common advice was that if you were going to store it for a couple of months to fill it completely to the top to prevent rusting. That was back when gas was good, tanks were metal, and sheep were scared. Fuel used to have rust inhibitors, lead, zinc, and other items that helped keep the valve train lubricated. Modern fuel doesn't have any of these and the new cleaners they add can cause bare metals to flash rust even sooner. Manufacturers were told to switch to plastic when modern fuel injection began mass production. The manufacturers of the injectors told the auto industry that any little speck of contamination could cause failure. That's why you see plastic tanks, submersed fuel pumps and inline filters installed inside the tanks shielded from the outside. Modern cars have plastic fuel tanks as well as about 80% of the new bikes.
I can't comment about whether you should drain your fuel or leave it. I can't tell where you post from. Well I could search your ISN but I'm lazy and you might be using a third party to generate a false ISN as many people do these days. It's the 10%+ ethanol that you need to worry about. If you get 100% gas and put some stabilizer in it, Startron comes highly recommended these days, then 4 months might be fine. Expect rough running for the first tank and then better performance with each new tank of fuel after hibernation.
09 Aprilia RS125 plated< blew that sum bitch up! New engine build soon. Rest in pieces sweet heart. 10/14/2016.
06 Paul Smart Sport Classic NFS ever.
08 S4RS Tricolore #081 NFS ever.
The pink Monster has moved away.
97 900SS CR
72 Norton Commando 750
03 KX60 with S4R rear shock,Honda CB350F, Hodaka Ace90, 3x Yam-YGS1, Yam-CS3 200, Vespa small frame, Gilera 106, Puch Sabre, Puch 50 Boy Racer, Benelli 250, Benelli 360, Honda CB350T, and many more.