There are a lot of differences between the 2.
The GC4 has flush pockets
The GC4 castings bolt sideways into the rail
The GC4 has 2 cross bars with no center bar
The GC4 frame is thicker and heavier
The GC4 has levelers built into the frame for planing. (No wooden wedges)
The GC4 has ball bearing leg pads which allow you to level the table by yourself with all the slate on it
Some GC3s will have pins connecting the slate. (Which mine does) GC4s don't. Which is good.