Tempered Glass Tabletop - Safe dimensions for a given base?
If you use only four support points, your glass top is actually sitting on a square about 2.12 feet on each side. So your overhang for a 4 foot circle is about 11 on each side.



Tempered glass is very strong (note that annealed or hardened glass is also strong, but much less so than tempered). This calculator suggests that a 1/2" tempered glass shelf 11" x 25" supported at the ends would withstand a weight of over 600 lbs. The pressure on your table edge is somewhat different, since it is supported only along one edge, but that edge is attached to the rest of the top. It sounds as if there is little danger of the unsupported edge breaking off.

The bigger risk is tipping. If someone leaned on the edge of the top in between two of the support points, you have a lever of about 1 foot lifting three fee of tabletop. The top would be quite heavy (maybe 80+ lbs.) and would take a lot of pressure to tip. But it is quite possible, say if someone sat on the edge.

Consider adding several more support points around the edge (at least 4 more).  This would increase the supported area to nearly 3 feet, and reduce the unsupported section to just over 6 inches.