If the required margin for hedged positions is 0, how can my positions be stopped out?

While there is no margin requirement requested for fully hedged positions, this does not protect your orders to be closed out at one point.

Mainly this is related to spread widening. Spreads may widen depending on the product you are trading during overnight hours, over news releases, during market opening and market closure, as the liquidity is thin, and the volatility is high.

This can also be related to the overnight financial charges, which apply to each position if you keep them open over rollover, which takes place at midnight MT4 platform time each trading day.

The moment the equity on your account falls below zero your open positions will be closed out, so it is important to ensure you have funds to support your account.