The 4piece bonus is worth getting. It is a dps increase.
Since the Lavaburst is instant, you will have finished casting it ~1.2 seconds earlier. During that time a lavasurge proc can occur and you can cast your next Lavaburst just after the first one. Without 4piece bonus that would not be possible. It does not happen very often in single target fights, but it does.
And you are right, the 4pice bonus is great for movement. Except for Beth'tilac and maybe Shannox every fight involves (heavy) movement. Dotting Shannox' dogs to get more Lavasurge procs is maybe a dps increase (overall for sure, on Shannox I am not so sure) and definitely more fun, seeing more Lavabursts flying around
Besides, what other option would we have, except for hands which is not that great an update. Chest, Legs and Shoulders are Set anyway.