Unless you're planning to move the lift alot, I would suggest bolting it down. Those lift posts take alot of strain when a vehicle is on them. I've seen what happens when a lift fails-not a pretty sight. If you need to move the lift however, I think that 4"x4" tube would be a better choice for bracing. The Rotary lift in the shop I work in has that size bar in the front and rear, it is bolted down also.
If you need a portable 4 post lift, they are available from Backyard Buddy. They are already braced and have casters on them for easy moving.
