Table
so i'll either need to build or put the jack on something in order for the jack to reach the bottom of the frame, correct? would you recommend 4 jacks then?
There is a few ways to shore up the table to place a wheeled dolly,sliders or installing a rug under the legs. GC 4 frame is known for the end frame to not hold tight so when you jack up the frame side to side it could shift popping the slate seams or screw holes, I'm not saying all GC4's have the issue but it is a possibility on your table. If its a drop pocket table it's a little easier and less of a chance of slate movement to jack it end to end with a green treated 4x4 spanned across "under" the end of the frame timber tails, be sure to get the jack centered using the rail aiming sight as a reference for center to place the jack under the 4x4 to frame.
The frame can be reinforced to fix a end sill sag but the table will need to come apart to do so.
Make sure you can screw the feet in and out all the way and make sure the threaded insert for the feet are secure in place, if not its a good time to do the repair while its jacked up in the air.
I've had good results using a GC simple lifting rig that RKC suggested.
You'd be shocked to know that just about everybody has the most important part needed to lift a GC.
If you jack the table side to side just go slow and only high enough to place a moving device under the feet.
Best of luck, take your time and be safe.
Rob.M
Edit: which reminds me, pat'O had a table jack "ez-jack" that I need to build a copy of if I can't find one