Safety has a whole lot more to do with the user than it does with the lift itself. 

Probably less vehicles falling off 4 post lifts than 2 post lifts but that doesn't mean the 2 post is more dangerous, it means the 4 post is more idiot proof. With a 2 post I like to lift the vehicle a few inches and then give it a good shake side/side, front/back, if it feels good it'll almost definitely be fine unless something in the work significantly changes the weight distribution (like dropping an engine/transmission). 

I have a 9k no-name 2 post and have been incredibly happy with it, for what it cost no other option came close. If I have a larger budget to spend I would have looked hard at a 12k or bigger 4 post lift with two rolling jacks.