What is the smallest mortise and tenon configuration the machine will make? Is it 4mm thick by 20mm wide by how long? Can this be used in wood as thin as 1/4" or must it be thicker?

Ideally, you would want to use 1/2" thick materials with the Domino and the 4mm tenons. The general rule of thumb is that the tenon should be roughly a third of the thickness of your material. 4mm comes out to about 5/32". The depth of the tenon is ~20mm. The actually mortise is slightly deeper than the length of the tenon by 1-2mm.