Dragon Wars by Becka Sutton

Weird events have beset the Stevens Quadruplets since infancy.  But nothing can prepare them for the day they open their door and find another world outside.  A world inhabited by intelligent mythical creatures, most of whom are hostile to humanity. And it seems they are expected to save it from disaster.

Even with the help of their bonded ‘heart friends’ can they save both this strange, new world and Earth when the people they are trying to help want to kill them?