Just a note to add...
Each full sus bike will have its own traits, and characteristics. Some are very good climbers, others not so, some great at descending, others not so. Trying to give "general" advice on how to ride a full suspension bike compared to a hardtail is quite hard, other than what Jedi said... ie. you have to react a bit earlier, and preload the bike on jumps.
Of course, the more suspension travel you have, the earlier you have to react, and the more preloading you have to do. With the Orange ST4 you've only got a 4" travel bike, so to be honest, as a first FS bike, it'll probably be great. If you jumped straight in with 7" of travel both ends, you may end up a bit overwhelmed.
What some people have said about the sitting down thing... Well... What full sus does in particular is allow you to get away with sitting down a bit more when you're tired. That is not to say you should be sitting down where you were standing up on your hardtail, but more that when you're tiring a bit, you can get away with sitting down over a few bumps whereas you wouldn't get away with it on a hardtail.
Oh, and some full sus designs bob more than others under climbing. I've not ridden an ST4 before so don't know, but on most full sus bikes pedalling "smooth circles" when climbing is much much better than just stomping on the pedals. And this is where sitting on the saddle helps a bit.