I have 27 x 40" x 1/2" sheet of glass that i want to turn into a patio table. The glass already has a 1/2" hole drilled at each corner. Plan is to use 4x4 redwood posts as legs. Question is, how to attach the legs but allow them to be removed later for transport?
You will not want to have the legs just individually attached to the corners of the glass. Instead you will want to make a frame as a type of apron board around between the legs to hold things together and to provide lateral bracing for the legs. Without that the leg attachment would tend to be transmitting any lateral forces into the glass which would not be a good or safe thing.
You can build this frame to be fully under the glass top or have the frame surround the glass edge such that the top lays into the frame.
For a frame that you would put under the top this picture shows the construction technique that allows the corner legs to be removable from the frame (bottom view):
The parts in the above picture appear to be just loose fitted or not done carefully enough to get a great fit. When I have made a frame like this I have used a corner block with square edges and notched it into the apron boards like this:
The fastener that you use into the leg is a combination threaded bolt and lag screw like shown below. The lag portion gets screwed into the leg.
For extra strength two of the leg bolts can be used as shown in this picture: