They will usually be rubbish, due to the fact they haven't been built for a specific purpose.
It doesnt have to be a dh 4x bike. alot of 4x racers use cross country full suspension bike with small frames due to them being very light. so if you get an all mountain bike (gt i drive 5 is nice, very good looking bike and has the important low top tube) and put longer forks eg pikes or all mountains, lyriks on it. a longer stoke shock can be used to increase travel to about 6" and will be great!
The wild card suggested above is also a good option