[Serum]

1, they can become unstable, when discharged below 3V per cell. Overcharging also can cause problems, if you use a proper balancer you can't go wrong with the charging process. I personally still charge my lipoly cells outside or in a concrete like (small) bunker.. Just in case..
you need to use a cutoff device with the discharging. (that 3V/cell thing)
2, no, the outside is of an isolating polymer.
3, that is one of the chargers you can use.
4, depends on the type of cell, normally a lipoly can be carged with 1C (1X their own capacity, so in this case 4000 mAh.)

The normal charge character of a lipo cell is they charge with 1C (4000 mAh (4A) ) till the 4.2 volts per cell is reached, after that, the current is turned back, and the voltage stays at about 4.2V. so with normal cells (nimh's) charging them with 1C, it takes 1 hour to charge, and with lipoly it takes about 1.25-1.5 hour.