farm jacks?

[donm449]

I've seen folks talk about using farm jacks to raise top. In order to raise the entire top, not just one corner, would you need two jacks. Placed on each side and in the middle, or four, one on each corner.

Next which length jack works better the 48" work or does the 60". Thanks...

 

[Fireballsocal]

4, 1 in each corner and I have seen pics of a longer hi-lo with 6 jacks being used.